Miscellanous maintenance tasks

[kandersolar]

This PR:

• further cleans up the README
• adds solarfactors Contributors to the license file
• drops versioneer, and replaces it with setuptools_scm
• drops unused dependencies, and lists missing ones

[kandersolar]

I compared the wheel file with the latest one on PyPI and it looked good to me. pvfactors.__version__ works as expected. Tests are passing too. So I think this is good to go. I will follow this with a PR finalizing the release so that the fixes in #22 are available on PyPI.