GitLab

On Fermi, store forces as 64 bit fixed point and update with atomic operations.  Also various other minor optimizations.

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

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.