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the System. The System inspects all of its forces and calls
usePeriodicBoundaryConditions -- it reports 'true' if at least one of its Forces
returns 'true' for usesPeriodicBoundaryConditions.

The default Force implementation is to raise an exception (NotImplementedError).
The System implementation will return true if at least one of its Forces returns
'true' for usesPeriodicBoundaryConditions, _regardless_ of whether or not one or
more of the forces does not implement the method. If at least one of the forces
does not implement usesPeriodicBoundaryConditions and all forces return false,
an OpenMMException is thrown.

Changed how kernels get initialized.  StandardMMForceField now has you set the 1-4 interactions explicitly.  CMMotionRemover allows the removal frequency to be set.