GitLab

* Use PocketFFT instead of FFTW

* Minor cleanup

* Use PocketFFT instead of fftpack for reference platform

* Remove FFTW as a dependency

* Converted a test case to use PocketFFT

* Fixed an incorrect comment

* Converted RPMD plugin to common platform

* Merged RPMD tests for different platforms

* Try to fix errors on CPU OpenCL

Platform specific header files get installed.  This allows plugins to be built with just an OpenMM installation, not a full source tree.

1) Added DLL export macros to many classes; enough to get tests to compile
2) Cosmetic changes to reduce warnings in Visual Studio:
   a) change many "for (int i ="... to "for (size_t i ="... to avoid unsigned mismatch warnings
   b) included many explicit static_cast<OpenMMReal>() casts to document places where double->float conversion warnings occur

Created a test case for StandardMMForceFieldImpl's findExclusions() method.  Fixed some bugs that it found.  Created a CMake script and configured it for building and running test cases.