Replace missing headers as needed

Python part still needs to be fixed, almost there, compiles in clang again

The key point is to speed up compilation, and make it easier to develop, by using the packaged C++ interface more extensively

Added multipolar terms; doesn't compile on mac anymore

Updated the pybind11 interface

Finally done

So inconsistent in naming...

Not sure it helps...

Attach model specific methods at model construction

Dropped three more esoteric ones with good replacements