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.
Maybe the right thing would be to add
`set (Boost_PYTHON_LIBRARIES ${Boost_Python_LIBRARIES)`
here.