Add support for reading Gromacs FEP topology files

[chryswoods]

While we can write Gromacs FEP topology files for merged molecules, we cannot read back those files. We should add support for reading Gromacs topology files that contain merged molecules.

[lohedges]

Just to say that this could entirely be done in the Python layer using the approach here. Obviously it would be better to do this directly in the GroTop parser, but it would easy for me to add something like the above to the BioSimSpace Python layer, i.e.:

• Load the input files as normal with readMolecules.
• If GroTop was detected as a file format, then try to split the top files into their end states and convert to AMBER format.
• Re-read the split topology with readPerturbableMolecules.

[chryswoods]

I'm bumping this to the next released as I haven't had time to think about it. I think you are right that a simple workaround approach may be easier.