Add correct electron-to-proton ratios and mean molecular masses for different abundance tables

[DavidT3]

Currently the only added electron-to-proton ratio is for Anders and Grevasse (angr), though I did write XGA so other values of e-to-p for other tables could be added (just never really did).

Figured out (I'm pretty sure) how to calculate e-to-p and mean mol weight:

https://gist.github.com/DavidT3/438bc58ed23fbbbb9f02c9d61f66248e

So will include that - those values assume a typical metallicity of 0.3 though, so maybe should add a function that calculates for an input tables and metallicity.

I don't think the mean molecular weight is currently set up to allow different values for different tables, so will need to change that.