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Mods to get Free energy plugin libs to compile on Windows; still problem w/ kCalculateLocalSoftcoreForces.cu
and hence OpenMMFreeEnergyCuda lib

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 

The differences in the Ewald energies between the Reference and Cuda platforms were traced to roundoff error in the 
computation of the Vdw and Coulomb ixns for the short-range energy contribution (verified using double precision in the sum); the vdw and Coulomb terms are now accumulated separately and then added to the total energy 

In Ewald SR portion of code, the LJ contribution to dEdR was missing a factor of (inverseR*inverseR)

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

Forced computation of dot product used in computing angles to be done in double precision; acos() is very steep for angles near pi and single precison was giving disagreement w/ corresponding Gromacs values (discrepancies of the order of ~1.0)  

Fixed some MSVC conversion warnings and moved method-local inclusion of pme.h up to start of LJ file -- it wouldn't compile when included locally due to attempt to match extern binding with anonymous struct type.

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