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In Ewald SR portion of code, the LJ contribution to dEdR was missing a factor of (inverseR*inverseR)

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

Forced computation of dot product used in computing angles to be done in double precision; acos() is very steep for angles near pi and single precison was giving disagreement w/ corresponding Gromacs values (discrepancies of the order of ~1.0)  

Fixed some MSVC conversion warnings and moved method-local inclusion of pme.h up to start of LJ file -- it wouldn't compile when included locally due to attempt to match extern binding with anonymous struct type.

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- 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.