Partial commit while building model interface.

CMakeLists.txt

python/bindings.py

python/test_model.py

src/multibody/joint/joint-variant.hpp

src/multibody/model.hpp

src/python/__init__.py

src/python/data.hpp

src/python/force.hpp

src/python/inertia.hpp

src/python/model.hpp

src/python/module.cpp

src/python/motion.hpp

src/python/python.cpp

src/python/python.hpp

src/python/se3.hpp

src/spatial/inertia.hpp

src/spatial/symmetric3.hpp