Ionbox refactor

[daviesje]

This is a coarse modularisation of the ionised box calculation which separates the large function into a number of smaller steps. This should make future development easier. I have tested that we get the same results under a few variations of parameters, and it has not slowed down.

Key differences:

• constants are calculated at the beginning and held in a struct which is passed into each function.
• Radii are explicitly calculated at the beginning of the run, instead of on-the-fly in a while loop
• the RNG for partial ionisations has been explicitly removed (it only affected the !USE_MASS_DEPENDENT_ZETA case)

Further modularisation is possible, but best left to another PR