Move Photon Conservation Model to Python

[daviesje]

The current implementation of the redshift-dependent photon conservation approximation spans the Python Wrapper and C backend in a very awkward way, relying on many globally defined arrays which have to be passed back and forth. This makes changes to the model very difficult.

Since the model has to run a calibration simulation from the wrapper, and otherwise consists of relatively light array operations, I see no reason not to migrate the entire model to the wrapper. In this way we can simply pass a structure containing all the photon conservation info into ionize_box.