Integrate pyc2ray as an optional IonizedBox compute function

[daviesje]

With the new halo sampler, we should be able to create a flag which substitutes the excursion set reionisation algorithms for model comparison tests. Michele Bianco has already done a lot of work in getting the halo-related outputs from 21cmFAST to work with pyc2ray, and we would like to implement this fully within the py21cmfast package. The main steps would be as follows:

• alter the compute() method of IonizedBox to optionally bypass the regular compute method, calling another function in the wrapper.
• this function would take a HaloField or HaloBox, convert it to the units expected by pyc2ray, pass the required fields to the GPU if needed, and call the ASORA RT algorithm. It would then populate the IonizedBox fields with those given by pyc2ray.

Calculating from the HaloBox would be much easier as we just need to pass in the n_ion and density fields, but to get a little more resolution on our sources (effectively a grid of DIM rather than HII_DIM) we could pass in the catalogs in HaloField after converting to ionising emissivity using the functions in HaloBox.c.

[daviesje]

Reading in the recombinations will Involve:

• separating the recombination call from the IonizedBox compute function so we can access it directly.
• pull out dNrec (not the field in IonizedBox which is cumulative) at each timestep.
• pass the instantaneous grid into the pyc2ray package.