Adding _ch to channel tag and _pl to planet tag in S5 variable names

[taylorbell57]

This PR reformats the fitted variable names in Stage 5 to use _ch# for different channels and _pl# for different planets. This will make the code far more robust since we can far more easily identify the channel and planet number, and this should also help with Stage 6 bugs. It will also make the variable names a bit more human readable, and this PR should also fix multiple bugs related to trying to manually set different priors or fixed values for different channels.

These changes resolve #643

While I was at it, I also added support for ecosw and esinw in astrophysical models which avoids the well known ecc>0 bias of fitting for ecc and w directly.

I still need to fully integrate and test the _pl tags, especially for starry, and I still need to do a bunch of testing. For now I'm just opening a draft PR to help me keep track of my progress, so don't worry about reviewing it until I mark it as ready for review.

Note, this change is going to break backwards compatibility for S6 running on old S5 outputs, but that isn't a huge deal since S6 isn't critical and we're working towards v1.0