make Charmm possible

[huichenggong]

When using Charmm, after the system is created by CharmmPsfFile.createSystem or ForceField.createSystem, there will be 2 forces controlling nonbonded. NonbondedForce handles electrostatics, and CustomNonbondedForce handles vdW. I add some code to include lambda+soft-core in the existing CustomNonbondedForce if it is detected.

[huichenggong]

I just realize this is not really safe, because the energy expression in the CustomNonbondedForce is not the same from 2 createSystem.
CharmmPsfFile.CreateSystem gives '(a/r6)^2-b/r6; r6=r^6;a=acoef(type1, type2);b=bcoef(type1, type2)'
and
ForceField.createSystem gives 'acoef(type1, type2)/r^12 - bcoef(type1, type2)/r^6;'
I will try to cooperate with both