make test_forecast.py more robust

[wholmgren]

Some of the forecast tests are not very reliable due to issues with the data feed (I think). Some combination of mocking, allowing failures, and more granular testing is needed in that module.

[wholmgren]

Some examples of the kinds of failures/errors that show up.

1. not isnull() assertion failures:

=================================== FAILURES ===================================
____________________________ test_process_data[NAM] ____________________________
model = NAM CONUS 12km from CONDUIT, best
    @requires_siphon
    def test_process_data(model):
        for how in ['liujordan', 'clearsky_scaling']:
            data = model.process_data(model.raw_data, how=how)
            for variable in _nonnan_variables:
>               assert not data[variable].isnull().values.any()
E               assert not True
E                +  where True = <built-in method any of numpy.ndarray object at 0x7f212b69cda0>()
E                +    where <built-in method any of numpy.ndarray object at 0x7f212b69cda0> = array([ True,  True,  True,  True,  True,  True,  True,  True], dtype=bool).any
E                +      where array([ True,  True,  True,  True,  True,  True,  True,  True], dtype=bool) = 2016-12-24 21:00:00-07:00    True\n2016-12-25 00:00:00-07:00    True\n2016-12-25 03:00:00-07:00    True\n2016-12-25 06:00...07:00    True\n2016-12-25 15:00:00-07:00    True\n2016-12-25 18:00:00-07:00    True\nFreq: 3H, Name: temp_air, dtype: bool.values
E                +        where 2016-12-24 21:00:00-07:00    True\n2016-12-25 00:00:00-07:00    True\n2016-12-25 03:00:00-07:00    True\n2016-12-25 06:00...07:00    True\n2016-12-25 15:00:00-07:00    True\n2016-12-25 18:00:00-07:00    True\nFreq: 3H, Name: temp_air, dtype: bool = <bound method NDFrame.isnull of 2016-12-24 21:00:00-07:00   NaN\n2016-12-25 00:00:00-07:00   NaN\n2016-12-25 03:00:00-07...0-07:00   NaN\n2016-12-25 15:00:00-07:00   NaN\n2016-12-25 18:00:00-07:00   NaN\nFreq: 3H, Name: temp_air, dtype: float64>()
E                +          where <bound method NDFrame.isnull of 2016-12-24 21:00:00-07:00   NaN\n2016-12-25 00:00:00-07:00   NaN\n2016-12-25 03:00:00-07...0-07:00   NaN\n2016-12-25 15:00:00-07:00   NaN\n2016-12-25 18:00:00-07:00   NaN\nFreq: 3H, Name: temp_air, dtype: float64> = 2016-12-24 21:00:00-07:00   NaN\n2016-12-25 00:00:00-07:00   NaN\n2016-12-25 03:00:00-07:00   NaN\n2016-12-25 06:00:00-07...00-07:00   NaN\n2016-12-25 15:00:00-07:00   NaN\n2016-12-25 18:00:00-07:00   NaN\nFreq: 3H, Name: temp_air, dtype: float64.isnull
pvlib/test/test_forecast.py:60: AssertionError

=================================== FAILURES ===================================
___________________________ test_process_data[HRRR] ____________________________
model = NCEP HRRR CONUS 2.5km, best
    @requires_siphon
    def test_process_data(model):
        for how in ['liujordan', 'clearsky_scaling']:
            data = model.process_data(model.raw_data, how=how)
            for variable in _nonnan_variables:
>               assert not data[variable].isnull().values.any()
E               assert not True
E                +  where True = <built-in method any of numpy.ndarray object at 0x7f127b246580>()
E                +    where <built-in method any of numpy.ndarray object at 0x7f127b246580> = array([ True,  True,  True, False, False,  True, False,  True, False,\n        True,  True, False,  True,  True,  True,  True], dtype=bool).any
E                +      where array([ True,  True,  True, False, False,  True, False,  True, False,\n        True,  True, False,  True,  True,  True,  True], dtype=bool) = 2016-12-02 18:00:00-07:00     True\n2016-12-02 19:00:00-07:00     True\n2016-12-02 20:00:00-07:00     True\n2016-12-02 21...:00     True\n2016-12-03 08:00:00-07:00     True\n2016-12-03 09:00:00-07:00     True\nFreq: H, Name: temp_air, dtype: bool.values
E                +        where 2016-12-02 18:00:00-07:00     True\n2016-12-02 19:00:00-07:00     True\n2016-12-02 20:00:00-07:00     True\n2016-12-02 21...:00     True\n2016-12-03 08:00:00-07:00     True\n2016-12-03 09:00:00-07:00     True\nFreq: H, Name: temp_air, dtype: bool = <bound method NDFrame.isnull of 2016-12-02 18:00:00-07:00          NaN\n2016-12-02 19:00:00-07:00          NaN\n2016-12-...\n2016-12-03 08:00:00-07:00          NaN\n2016-12-03 09:00:00-07:00          NaN\nFreq: H, Name: temp_air, dtype: float32>()
E                +          where <bound method NDFrame.isnull of 2016-12-02 18:00:00-07:00          NaN\n2016-12-02 19:00:00-07:00          NaN\n2016-12-...\n2016-12-03 08:00:00-07:00          NaN\n2016-12-03 09:00:00-07:00          NaN\nFreq: H, Name: temp_air, dtype: float32> = 2016-12-02 18:00:00-07:00          NaN\n2016-12-02 19:00:00-07:00          NaN\n2016-12-02 20:00:00-07:00          NaN\n2...N\n2016-12-03 08:00:00-07:00          NaN\n2016-12-03 09:00:00-07:00          NaN\nFreq: H, Name: temp_air, dtype: float32.isnull
pvlib/test/test_forecast.py:60: AssertionError

2. an error related to the time index of the data:

==================================== ERRORS ====================================
__________________ ERROR at setup of test_process_data[NDFD] ___________________
request = <SubRequest 'model' for <Function 'test_process_data[NDFD]'>>
    @requires_siphon
    @pytest.fixture(scope='module', params=_modelclasses)
    def model(request):
        amodel = request.param()
        amodel.raw_data = \
>           amodel.get_data(_latitude, _longitude, _start, _end)
pvlib/test/test_forecast.py:51: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pvlib/forecast.py:252: in get_data
    self.data = self._netcdf2pandas(self.netcdf_data, self.query_variables)
pvlib/forecast.py:333: in _netcdf2pandas
    self.set_time(netcdf_data.variables[time_var])
pvlib/forecast.py:357: in set_time
    times = num2date(time[:].squeeze(), time.units)
netCDF4/_netCDF4.pyx:3695: in netCDF4._netCDF4.Variable.__getitem__ (netCDF4/_netCDF4.c:37923)
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
>   ???
E   IndexError
netCDF4/_netCDF4.pyx:4374: IndexError

3. a netCDF4/HDF5 library import error that randomly occurs on different python versions: (fixed in #293)

==================================== ERRORS ====================================
_________________ ERROR collecting pvlib/test/test_forecast.py _________________
ImportError while importing test module '/home/travis/build/wholmgren/pvlib-python/pvlib/test/test_forecast.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
pvlib/test/test_forecast.py:23: in <module>
    from pvlib.forecast import GFS, HRRR_ESRL, HRRR, NAM, NDFD, RAP
pvlib/forecast.py:6: in <module>
    from netCDF4 import num2date
../../../miniconda/envs/test_env/lib/python3.5/site-packages/netCDF4/__init__.py:3: in <module>
    from ._netCDF4 import *
E   ImportError: libmfhdf.so.0: cannot open shared object file: No such file or directory

No idea what to do about that other than hope it's a problem with the upstream packages and that it will fix itself soon.

4. Sometimes a data request will time out. This definitely happens with HRRR_ESRL. Not sure about others. (fixed in #293)