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.

examples/EAGLE_ICs/EAGLE_25/eagle_25.yml

@@ -190,15 +190,23 @@ EAGLEFeedback:
 
 # EAGLE AGN model
 EAGLEAGN:
-  subgrid_seed_mass_Msun:           1.5e5      # Black hole subgrid mass at creation time in solar masses.
-  max_eddington_fraction:           1.0        # Maximal allowed accretion rate in units of the Eddington rate.
-  eddington_fraction_for_recording: 0.1        # Record the last time BHs reached an Eddington ratio above this threshold.
-  with_angmom_limiter:              1          # Are we applying the Rosas-Guevara (2015) viscous time-scale reduction term?
-  viscous_alpha:                    1e6        # Normalisation constant of the Bondi viscuous time-scale accretion reduction term
-  radiative_efficiency:             0.1        # Fraction of the accreted mass that gets radiated.
-  coupling_efficiency:              0.15       # Fraction of the radiated energy that couples to the gas in feedback events.
-  AGN_delta_T_K:                    3.16228e8  # Change in temperature to apply to the gas particle in an AGN feedback event in Kelvin.
-  AGN_num_ngb_to_heat:              1.         # Target number of gas neighbours to heat in an AGN feedback event.
-  max_reposition_mass:              2e8        # Maximal BH mass considered for BH repositioning in solar masses.
-  threshold_major_merger:           0.333      # Mass ratio threshold to consider a BH merger as 'major'
-  threshold_minor_merger:           0.1        # Mass ratio threshold to consider a BH merger as 'minor'
+  subgrid_seed_mass_Msun:             1.5e5      # Black hole subgrid mass at creation time in solar masses.
+  multi_phase_bondi:                  0          # Compute Bondi rates per neighbour particle?
+  with_angmom_limiter:                1          # Are we applying the Rosas-Guevara et al. (2015) viscous time-scale reduction term?
+  viscous_alpha:                      1e6        # Normalisation constant of the viscous time-scale in the accretion reduction term
+  radiative_efficiency:               0.1        # Fraction of the accreted mass that gets radiated.
+  max_eddington_fraction:             1.         # Maximal allowed accretion rate in units of the Eddington rate.
+  eddington_fraction_for_recording:   0.1        # Record the last time BHs reached an Eddington ratio above this threshold.
+  coupling_efficiency:                0.15       # Fraction of the radiated energy that couples to the gas in feedback events.
+  AGN_delta_T_K:                      3.16228e8  # Change in temperature to apply to the gas particle in an AGN feedback event in Kelvin.
+  AGN_num_ngb_to_heat:                1.         # Target number of gas neighbours to heat in an AGN feedback event.
+  max_reposition_mass:                2e8        # Maximal BH mass considered for BH repositioning in solar masses.
+  max_reposition_distance_ratio:      3.0        # Maximal distance a BH can be repositioned, in units of the softening length.
+  with_reposition_velocity_threshold: 1          # Should we only reposition to particles that move slowly w.r.t. the black hole?
+  max_reposition_velocity_ratio:      0.5        # Maximal velocity offset of a particle to reposition a BH to, in units of the ambient sound speed of the BH. Only meaningful if with_reposition_velocity_ratio is 1.
+  min_reposition_velocity_threshold: -1.0        # Minimal value of the velocity threshold for repositioning [km/s], set to < 0 for no effect. Only meaningful if with_reposition_velocity_ratio is 1.
+  set_reposition_speed:               0          # Should we reposition black holes with (at most) a prescribed speed towards the potential minimum?
+  threshold_major_merger:             0.333      # Mass ratio threshold to consider a BH merger as 'major'
+  threshold_minor_merger:             0.1        # Mass ratio threshold to consider a BH merger as 'minor'
+  merger_threshold_type:              2          # Type of velocity threshold for BH mergers (0: v_circ at kernel edge, 1: v_esc at actual distance, with softening, 2: v_esc at actual distance, no softening).
+  merger_max_distance_ratio:          3.0        # Maximal distance over which two BHs can merge, in units of the softening length.
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