GitLab

Moved SFMT into libraries folder, and restructured it to allow multiple independent random number generators

to avoid running OpenCL tests on build machines with OpenCL installed, but no GPU.

Unify FindOpenCL.cmake files to one location, under cmake_modules.
Other changes to permit building with nvidia opencl on Windows.

All platforms now handle nonbonded exceptions in a consistent way (they ignore cutoffs and periodic boundary conditions).  Also started OpenCL implementation of CustomHbondForce.

Increased the tolerance on two tests that started failing when we switched the reference platform to double precision

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