GitLab

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

Incorporate Normal Mode Langevin openmm code from Chris Sweet as a plugin.  (compiles on windows, but no actual testing done)

Added a version of Platform::getPlatform() that looks up platforms by name.  Added a Context constructor that allows platform-specific properties to be specified.

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.