Do we need to be able to generate MGXS files for photons

[shimwell]

Description

I'm currently using the model.convert_to_multigroup method to make MGXS files for use in random ray FW-Cadis workflows.

The three available methods (stochastic_slab, infinite_medium and material_wise, all use neutron sources to make neutron MGXS cross sections.

Do we need to be able to generate MGXS for photon only transport in order to get FW-Cadis weight windows for shut down dose rate simulations. Keen to get your thoughts @jtramm

Alternatives

Perhaps photon transport is quicker so this is not as necessary

Compatibility

perhaps we add an argument for particle type and it can default to neutron to keep the same behaviour