GitLab

Added missing required kernel names (ApplyConstraintsKernel and VirtualSitesKernel) to check for platform capabilities, since these kernels are initialized in the next code block.

Separated hardware query functions into a separate file.  CPU platform checks for the correct hardware.

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

Converted the routine for identifying molecules from a recursion to a loop.  This avoids stack overflows with very long linear molecules.