Add support for CMAP terms and chamber files

[chryswoods]

Issue for myself to remind me that we need to add support for CMAP terms and chamber files (and the CHARMM forcefield generally) to the AmberPrm parser.

This will be particularly important as use of BioSimSpace and somd (or somd2) grows.

[lohedges]

We do provide some support for the CHARMM force field via Sire.IO.CharmmPSF. However, as with our other parsers, this also currently lacks support for CMAP records.

[chryswoods]

Cool - that's less work to do ;-)

I'll just need to add this to AmberPrm then.