ABFE net charge support

This is the overaching issue for adding net charge support in the OpenMM ABFE Protocol.

Net charge support will be added through the use of a co-alchemical ion that is annhilated alongside the ligand.

Some details:

• In the first instance an ion of the opposite charge will be co-annhilated. We are aware of previous reports that this doesn't always close the thermodynamic cycle properly with lipids. However recent reports seem to indicate that this behaviour is not always seen. For ease of implementation, we will start with co-annhilation and then explore charge transfer once that is finished.
• The transformation will be done with full nonbonded exclusions between the alchemical species, in a manner similar to SepTop. This should avoid some ligand-ion interaction issues as well as make it a lot easier to implement code-wise.