Hail2 python docs style

[tpoterba]

I suggest Numpy style docs: http://www.sphinx-doc.org/en/stable/ext/example_numpy.html#example-numpy

I think it’s really between this or google style: http://www.sphinx-doc.org/en/stable/ext/example_google.html#example-google

Thanks to Ben W for the nudge here.

[tpoterba]

These two PRs use numpy style docs:

Hail2 docs PR: move function docs to functions.py

Add docs to classes and methods in expression.py