Clipping sphinx documentation

[kperrynrel]

Description

This PR generates documentation for the clipping mask in pvlib.features. I added an example data set labeled for clipping, and compare the results of the mask to the ground-truth labeled data (taken from the Duramat data sets site). Related to issue #133.

Checklist

The following items must be addressed before the code can be merged.
Please don't hesitate to ask for help if you are unsure of how to accomplish any of the items.
You are free to remove any checklist items that do not apply or add additional items that are
not on this list

• Clearly documented all new API functions with PEP257 and numpydoc compliant docstrings
• Adds description and name entries in the appropriate "what's new" file
  in docs/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`).
• Non-API functions clearly documented with docstrings or comments as necessary
• Pull request is nearly complete and ready for detailed review

[kandersolar]

I'm not seeing a thumbnail for this example in the RTD PR build and I'm not sure why. It works when I build locally, so I'm inclined to chalk it up to browser strangeness and ignore it here.

The 4.7 MB data file is a bit large. Our current distribution is only 80-90 kB (https://pypi.org/project/pvanalytics/#files), so adding this would increase the size by 50x! What do you think about cutting down that data file to just the period used in this example and/or removing the unnecessary columns?

[kandersolar]

LGTM!

[cwhanse]

Looks good @kperrynrel . I would suggest using more descriptive legends, so that the two plots can be more easily identified as the labeled data and the detected clipping times.

[kperrynrel]

@kanderso-nrel and @cwhanse made all the updates suggested. If you guys are good with it, I'll merge the PR into master. Thanks for the help with reviewing this!