Document MCEq install process and/or add to setup.py

[jllanfranchi]

MCEq code is referenced in mceq service but must be installed manually by the user. This needs to be noted in the PISA install docs at least as an optional dependency, but ideally would also would be automated via the setup.py script (maybe with an additional keyword, so you don't have to install, but can install optionally like Numba, CUDA, etc.?).

[steven-j-wren]

I would vote for it being optional, but that's based on my personal anecdote of "that one time I tried to install it 2 years ago and it was a pain"

[LeanderFischer]

Installing MCEq is super simple now, by just pip install MCEq. But it would be nice to clean up the remainders of MCEq build related comments in the setup.py and add it to the requirements and test the install!