Strict separation of C++ and Python code bases

[Prof Garth Wells]

With the pybind11 change, we should enforce a strict separation of the C++ and Python interfaces, including:

• Build systems
• Testing
• Demos
• Documentation

This will simplify the code substantially by using the 'standard' tools for each case without ad-hoc modification and cross-linking Presently, there is an awful lot of unnecessary cross-over which is very tangled.