Python 3 support

Trying to build on top of smtk with python3, I get an error during cmake configure:

CMake Error: install(EXPORT "rgg-session-plugin" ...) includes target "smtkRGGSimulationProteusPlugin" which requires target "smtkRGGSimulationProteus" that is not in the export set.

most of proteus is python, and looking at cmake cache, it looks like there are some python2 libs set when the exe is python3.

@ben.boeckel any chance you could take a quick look?