Get documentation up to specs and caught up

[mrshirts]

What does the pymbar4 documentation need to look like? This includes:

• Docstrings
• What else?
• improve https://pymbar.readthedocs.io/en/master/? How?

[mrshirts]

After deciding what needs to be done, we will allocate who will do what. I imagine that somebody else will put the framework in shape, and I will fill in most of the content.