Add variable mapping to SRML iotools functions

[AdamRJensen]

• I am familiar with the contributing guidelines
• Tests added
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
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This PR adds the map_variables keyword to the pvlib.iotools.get_srml and pvlib.iotools.read_srml functions. These functions already were mapping the variables, but this PR gives the option of setting map_variables=False.

The main driver for this PR was to make the functions follow the standard pattern and to change the mapping of wind direction to wind_direction where it was previously mapped to wind_dir.

[kandersolar]

LGTM, just needs a whatsnew entry