adding support for numpy.ndarray and numpy.datetime64 to helpers.subsol

[gregstarr]

Same as #27 except between develop branches

[gregstarr]

Looks like all the CI tests failed, but I was able to get most of the tests to pass in windows subsystem for linux

[aburrell]

Ok @gregstarr this looks pretty good. I made some changes:

1. fixed the failing unit tests not related to your code. I expect Travis to pass, since I was able to get the same tests to fail locally and fix them.
2. made a few style changes. Obviously the contributing docs need to be improved to assist future contributions, but there's a lot here I need to get on!
3. updated the changelog

Anyway, the core elements of your contribution look great and the additional unit tests are appropriate. Before approving I need:

• Travis to pass (appveyor is failing due to changes in the appveyor system, and you tested windows)
• you to add your name to the end of AUTHORS.rst
• you to update the subsol docstring so that the Parameters description reflects the new allowable input.

[gregstarr]

I added to the subsol docstring and to AUTHORS.rst but I can't get the tests to pass on Windows 10. Basically all tests fail with the error: AttributeError: function 'cofrm_' not found. It looks like a compilation problem because the compiled file '.conda\\envs\\apexpy\\lib\\site-packages\\apexpy-1.0.3-py3.8-win-amd64.egg\\apexpy\\fortranapex.cp38-win_amd64.pyd' does not have cofrm_ in it. I will take a look, but I don't have much experience with this type of thing.

[aburrell]

Ok, since that problem is not related to this PR, I am going to merge it in.