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Plugins work with CPU platform.  This involved separating the reference implementations of plugins into their own libraries.

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.