remove irradiance parameter from ModelChain.run_model and ModelChain.prepare_inputs

[alamathe1]

pvlib python pull request guidelines

Thank you for your contribution to pvlib python!

You may submit a pull request with your code at any stage of completion, however, before the code can be merged the following items must be addressed:

• Closes remove irradiance parameter from ModelChain.run_model #488
• 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.

Please don't hesitate to ask for help if you're unsure of how to accomplish any of the above. You may delete all of these instructions except for the list above.

Brief description of the problem and proposed solution (if not already fully described in the issue linked to above):

[wholmgren]

Please also add a note to the api section of docs/sphinx/source/whatsnew/v0.6.0.rst as well as your name or username

[wholmgren]

The lines from "# The following part could be removed together with the irradiance" through "# **** End ****" should also be deleted.

[wholmgren]

Thanks @alamathe1! I'll merge this in a few days if there are no other comments. The test failures are unrelated.

[alamathe1]

@wholmgren Alright thanks!

[wholmgren]

Thanks again @alamathe1!