GitLab

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

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

Refactored ForceImpl, lots of KernelImpl subclasses, and other related classes to avoid redundant calculations when requesting a State with both forces and energies

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

Restructured how force and energy computations are done to make it easier to write plugins for the CUDA platform