[BUG] custom_parameters option not exposed via AMBER protein force field generator

[lohedges]

The new custom_parameters keyword argument isn't exposed to AMBER protein force field generator _make_amber_protein_function, so is missing from the functions for these force fields.

[lohedges]

It also appears that users want to source other built-in leaprc files when parameterising with a different force field. for example they may wish to add source leaprc.gaff when using a protein force field. This causes some issues with the current API since custom_parameters assumes a list of files, which are then validated, i.e. we make sure that the files exist then add loadParams directives for them. I'm not sure whether to add another option for leaprc files, or just check for leaprc in the name, then validate them as we do elsewhere? (As expected, it's a pain to support all possible use cases for the LEaP script via a few keywords.)