Add [10.1021/jacsau.4c00276] polymers to the polyply library

[ricalessandri]

Contributing the 3x phthalimide-containing polymer models (PMAP, PEPP, PVBP) developed in this recent paper to the polyply library.

As part of this, I'm also starting to use common termini, e.g., the same CH3 termini.

Things left:

• add new polymers to the library md doc
• remove the previous CH3.ff and update the PTMA model so that it uses the new one; probably, we'll have then to update the PTMA test
• add tests for the new polymers

[fgrunewald]

Ideally we would only have one CH3ter and change the charges or atomtype once it get's connected via a different residue / bond. Do you think that would be possible?

[ricalessandri]

Right, that makes sense. Does #327 handle this automagically? My itp_to_ff version does not. I could do it by hand for these 3 polymers.

[fgrunewald]

@ricalessandri yes, but there are some edge cases I haven't considered yet. For example, when CH3 is bound to acrylate either via the backbone or via the oxygen.