GitLab

Lots of new optimizations in ParsedExpression.optimize().  Also optimized CUDA evaluation of expressions.

All energies existing in Preview Release 3 now run on the GPU, new Ewald energy terms still use Reference implementation

Created a CudaPlatform property for specifying whether CUDA should sync or spin loop while waiting for results

- MingW, where "WIN32" is set by CMake but the compiler is not Microsoft's (should use _MSC_VER) in that case, and 
- in gcc 4.4 which is fussy about some issues that earlier versions were silent about, such as passing a string constant to a "char*" rather than a "const char*".
I also fixed a syntax error in CMakeLists that showed up in the OpenMM workshop for some unknown reason.

Brook is not currently building correctly; I think it has not yet been fully converted to the getTime() interface needed for variable step integrators.

Optimization to constraint algorithm: skip the last iteration as soon as we see that all constraints have converged.