GitLab

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

Added a version of Platform::getPlatform() that looks up platforms by name.  Added a Context constructor that allows platform-specific properties to be specified.

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