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When converting to fixed point, multiply by 0x100000000 instead of 0xFFFFFFFF.  This should be (very very slightly) more accurate, since its reciprocal can be exactly represented in floating point.

Began implementing new mixed precision model that does integration in double precision and force evaluation in single precision

Continuing to implement new CUDA platform: constraints, LangevinIntegrator, BrownianIntegrator, VariableLangevinIntegrator, VariableVerletIntegrator