WIP: Merge black hole (and potentially some other) updates from Yannick's development branch into master

This merge request integrates the changes made in my (primarily) black hole development branch. There are some more experimental features in there that we likely do not want to include, so feel free to pick (and adapt) what seems appropriate (a copy of this development branch is kept at yb-bhdev).

The main elements are:

• Options to fine-adjust many elements of the black hole model via YAML parameters
• Option to use black hole subgrid masses from (idealised) initial conditions
• Option to use "multi-phase" black hole accretion approach
• Option to prescribe a repositioning velocity for black holes
• Slight re-design of the black hole // gas interactions, so that gas neighbour velocities are always recorded in the black hole frame
• Some extra black hole outputs (which, as discussed previously, we may not want to take over to master)
• Crude option to use different levels of output (e.g. snap- and snipshots), and modify the output list upon restarting from restart files
• Option to easily block entire particle types from outputs
• Some (hopefully) clarifications and improvements to the comments

Also (added late):

• Changed the black hole output 'CumulativeNumberSeeds' to 'CumulativeNumberOfSeeds'. I kept getting this one wrong, since it is not quite consistent with the general convention of using unabbreviated, descriptive names. Feel free to ignore if there was a good reason for the original name.