Complete the XraySourceBox output

[daviesje]

The new halo model introduced a new output structure XraySourceBox, which contains the filtered star formation rate densities required to calculate the xray and lyman alpha heating fields in spin_temperature. This was separated from the main spin temperature code in order to support the use of source fields directly from each required redshift, rather than relying on linear growth which was impossible for the halos.

I would like to extend this functionality to the non-halo model, using the PerturbedField at the actual source redshift, and also supporting the new spherical shell filter, which directly ties the radiation received by a cell to the distance and redshift of the sources. This would also allow us to further clean up the ComputeTs function by combining the generation of the filtered SFR grids in the halo-based and density grid-based calculations.

The implementation of the boxes themselves also needs to be cleaned up a little, since it requires messing with some of the private attributes in order to allow multiple computations at different redshifts.