documentation fixes

[wholmgren]

Fix a handful of issues with the documentation.

Readthedocs has not built the documentation on master for the last couple of months. This PR should fix that. I think the major thing that broke the docs was IPython 7 becoming available and it being less tolerant of warnings and errors.

Here's a link to the built documentation on my fork: https://wholmgren-pvlib-python-new.readthedocs.io/en/docfix/

I'll merge in a few days if I don't hear any feedback.

• no issue to close
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes LGTM and SticklerCI checks.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

[cwhanse]

Redundant line here

[wholmgren]

@cwhanse thanks for the review.

[wholmgren]

Confirmed that documentation successfully builds again. The issue with some of the IPython output being hidden (and inputs all labeled [1]) has also disappeared (first seen in #568). I suspect that they are related.