ModelChain.prepare_inputs fails to pass solar_position and airmass to Location.get_clearsky

[wholmgren]

• Closes ModelChain.prepare_inputs fails to pass solar_position and airmass to Location.get_clearsky #481
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests must pass on the TravisCI and Appveyor testing services.
• Code quality and style is sufficient. Passes git diff upstream/master -u -- "*.py" | flake8 --diff and/or landscape.io linting service.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• 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.

Not sure how to test this or if it's worth the time investment. I recommend double checking the code is right and merging.