GitLab

Created API and reference implementation of restricting CustomNonbondedForce to selected interaction groups

Platform specific header files get installed.  This allows plugins to be built with just an OpenMM installation, not a full source tree.

When computing kinetic energy, make sure the shifted velocities are perpendicular to the constraints

Variable time step integrators correctly report the size of the last step taken.  Also improved accuracy of integration unit tests.

NonbondedForce sometimes included energy terms that weren't appropriate to the specified force groups