Move python bindings (kimpy) into kim-api package

[ellio167]

This adds a pybind11 dependency (but that can be managed reasonably with options).

The bigger question, for me, is install behavior. It would be good to find examples of other stand-alone (non-python) system libraries that also ship with a python wrapper package to use as guidance for how to do this in "the correct way".

Beyond the basic CMake install behavior and options, there is also the question of how to correctly handle binary packaging in the various systems we support.