Absolute solvation free energies + base things for ABFEs

[IAlibay]

Supersedes #237

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[IAlibay]

Putting the docs in the py file here probably lessens the merge conflict and is the route we’re taking though

So what I've done here now is included as many of the changes I could do that were done for the RFE protocol in #570 without completely breaking the docs (i.e. if we were to merge this today, you still have docs that work). Either in #570 or after its merge, we would then have to make the rest of the changes here (mainly just deleting out the rst file and importing the settings into the relevant file being added in #570).

[IAlibay]

Ok, as long as CI comes back happy, I think I've done everything on my side of the to-do list, i.e. planned tests and features