Add a salt concentration term to the custom GB forces

This introduces a "kappa" keyword that is related to the ionic strength, introducing a way to add the screening effects of counterions to GB simulations.

This change is only made to the GB models implemented as CustomGBForces inside the internal/customgbforces.py file of the Python application layer.  It also exposes this parameter via the createSystem method call in amberprmtopfile.