Further ease writing custom slice sources

[forman]

Is your feature request related to a problem? Please describe.

Within zappend, a slice item, either given as file path or URL or as custom slice source, is converted into a context manager and then used as such (https://github.com/bcdev/zappend/blob/main/zappend/processor.py#L98). Writing custom slice sources could therefore be further simplified if a slice item is a context manager.

Describe the solution you'd like

Allow slice items to be context managers, that is, they implement contextlib.AbstractContextManager or they provide the dunder methods __enter__() and __extit__(). Slice context managers must yield a xarray.Dataset - the slice dataset. Note, this excludes slice items that are already a xarray.Dataset, because xarray datasets are themselves context managers.

With this extension, a custom slice source can be written as follows:

from contextlib import contextmanager
import xarray as xr

@contextmanager
def get_dataset(ds_path, unused_var_names):
    # Code to acquire a dataset from somewhere
    # and optionally bind other resources.
    ds = xr.open_dataset(ds_path)
    try:
        # Further process dataset
        ds_subset = ds.drop_vars(unused_var_names)
        # Return processed dataset
        yield ds_subset 
    finally:
        # Close original dataset:
        ds.close()
        # Optionally release other resources

EDIT

Two additional settings could simplify writing of slice sources and make passing of parameters easier and more comprehensible:

1. slice_source_kwargs to pass extra keyword arguments to every slice source together with its slice item
2. extra to pass arbitrary configuration to a slice source. A syntactical alternative to keyword arguments passed in every call.