Fix for requests redirect_cache issue

[jwoillez]

Addresses #434

[keflavich]

Can you explain the motivation for changing __session to _session? Also, what was the motivation for __session in the first place?

[jwoillez]

The initial motivation for the private __session was really to hide it as much as possible from the developer. Nothing more.

NB: I have no way of testing the clean-ups I made on the commits above (poor internet connection).

[keflavich]

OK, I'll test before merging

[jwoillez]

Fingers crossed, then!

I also had some invasive changes on caching and on retrieve_data(), but lost them with a wrong git checkout. Caching seemed like the most problematic. There is a mix of suspend_cache() and cache=True/False throughout the code. We should settle on only one mechanism.

I will open issues for both. We can discuss there...

[keflavich]

Passed locally.