GitLab

Use a consistent (and more accurate) value for electric constant everywhere.  Also cleaned up handling of constants and units in various places.

Redesigned CustomNonbondedForce to eliminate combining rules and allow the expression to directly use parameters for individual particles.

Free energy plugin added
Plugin will not run w/ Obc or GB/VI forces unless line 2004 of gpu.cpp (gpu->sim.totalNonbondOutputBuffers  = 2*gpu->sim.nonbondOutputBuffers;) is commented in -- working on removing this constraint
Also unit tests for GB/VI currently fail 

Restructured how force and energy computations are done to make it easier to write plugins for the CUDA platform

All energies existing in Preview Release 3 now run on the GPU, new Ewald energy terms still use Reference implementation

In calcForces() kReduceBornSumAndForces kernel is not needed since Born radii/Obc chain term now calculated prior to calculation of forces
Modified kCalculateObcGbsaBornSum kernel loop to match that in kCalculateCDLJObcGbsaForces1; Born sums are zero w/o mods