Implement variable AGN heating temperatures

@matthieu I have mostly made this MR at this stage so we can have a place to look at the implementation of what we have been discussing recently. I don't think we need to merge this in until we've settled on what it should actually look like. I have a local version of this (for the COLIBRE fork) so I can already use it for tests.

A very similar implementation of this should be done for the jets, although I have left that untouched for now. We still need to talk a bit about the jet case.

One missing thing still (this is mostly a reminder for myself): I need to add descriptions of the new parameters in the relevant param files and in the documentation.

Finally I had a specific question about something I'm unsure about. I define the minimum hot gas sound speed from a minimum temperature through

bp->hot_gas_sound_speed_min = sqrtf(hydro_gamma * hydro_gamma_minus_one *
                        hot_gas_temperature_min * bp->temp_to_u_factor)

I am unsure about these prefactors involving gamma here. hydro_gamma is the standard prefactor for the sound speed, but then I also multiply with hydro_gamma_minus_one, because bp->temp_to_u_factor has the inverse of hydro_gamma_minus_one in its definition. Anyways, I am not sure if this is right.