Improve py4py documentation

[smangham]

A few small issues with the py4py reverb docs:

• The continuum argument for TF creation doesn't specify it should be central source + disk luminosity.
• The keplerian argument for TF.plot doesn't explain it should be in r_ISCO.

[smangham]

The threshold argument for TransferFunction.delay doesn't mention it's fractional. It should also mention .8 was the 'standard', ideally referencing the paper I got that from (which I can't remember right now!).

[smangham]

TransferFunction.plot is designed to chain and returns the TransferFunction on completion. It should have an option to return the actual figure created, in case users want to modify it further.

[smangham]

I forgot I still had a large script called "py4py,.py" in there that I was in the process of breaking up into smaller ones. I'll check this. Also, at some point (???) I decided to setup smangham/py4py separately from this repo. Presumably I was going to maintain it as its own thing then gave up and folded it in. I'll delete that repo.

[smangham]

This is ready, but being held up by Sphinx dependency hell. For some reason we had docutils pinned to <0.18? Anyway, builds now, looks fine.