Python3

Create an option (USE_PYTHON3 cmake switch, which is OFF by default) to build python3 bindings.

This does not check for the python3 boost component in externalpackages.cmake, since it is checked in src/python/CMakeLists.txt anyways (and soft-fails if not found). I don't see a reason for checking twice.
I'm changing this to Boost_Python_LIBRARIES, to match the capitalization that was used before (and which at least one site relies on for setting the variables custom).