azure pipelines config

[wholmgren]

Goal is to get both linux and windows building on azure pipelines and drop appveyor and eventually travis.

• Closes appveyor builds failing due to solarposition with numba warnings #713
• 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.

[wholmgren]

Ready to merge.

Azure Pipelines often completes 4 linux + 4 windows builds in under 6 minutes. This is less than 50% of TravisCI's time (albeit with 2 fewer linux configurations) and somewhere between 10-20% of the Appveyor's time (depending on how long until Appveyor actually gets started).

I'll turn off the Appveyor integration and delete .appveyor.yml once this is merged.

Dropping Travis is a little more complicated. Our Travis config has some special steps for the python 2.7 min build and I'd like to leave that alone until we drop 2.7. It also has a deploy key for pypi. Let's reevaluate in a few months.