Re-vamped packaging and C++ API contribution

[christopherbate]

Hi,

I have been working on updating the simulator to use Mujoco 2.3. Along the way, I have created a build system using CMake and wrote Pybind11 bindings instead of using the current ctypes approach. I think this makes the code more easily re-distributable as a Python package. I revamped the code significantly to use a C++ interface for the PyBind11 bindings, essentially moving the code in cassiemujoco.py into the C++ side. Not all functionality is working currently, but I wanted to gauge interest in contributing these changes back to this repo. In particular, these changes would be 1) use of C++ 2) use of CMake over Make and 3) use of PyBind11 for the Python bindings.

Thanks!