GitLab

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

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

Converted the array containing atom block indices for the neighbor list from ushort2 to int.  This removes the hard limit of 2 million atoms.

Integrators control when atom reordering gets done, instead of having it happen automatically when forces are evaluated.  This simplifies some code, and also improves unintuitive behavior with CustomIntegrator.