Colibre-specific AGN updates

This merge request implements two Colibre-specific updates to the AGN model: (i) Sound speeds are calculated based on particles' subgrid density; (ii) Bondi accretion rates are calculated for each neighbour individually and then summed to get the total accretion rate of the BH.

These changes are also implemented in branch yb-bhdev, together with other (more experimental) variations. There, they can individually be switched on/off with YAML parameters.