Simplify the construction of i/o properties and add the option to have an i/o conversion function returning an int for all the part types.

Lock before removing particles

See merge request !858

Re-check that a particle has not already been removed by a different thread in the locked section of the code when particles move out of their box.

Modified the default maximal Eddington fraction in all the EAGLE example to 150% to reflect the mistake from the original EAGLE runs where the fraction was set to 100% / h.

Version that correctly flags the BHs to swallow and removes them before dying in the time-step calculation function.

In the black hole interactions functions, added a loop over BH pairs that is only activated in the case of swallowing/merging.

Update the gpart mass when synchronizing when the star or BH mass changes.

Closes #592

See merge request !855

Update the black_holes_struct.h file to contain a structure for the BHs and added functions to manipulate the structure.

Move the part-related swallowing functions to the black_holes_struct file. Added a structure for the merging of BHs.

The old names were copied from the star routines and hence non-ideal.

Fix missing factor of h^3 when computing the BH swallowing probability in EAGLE's runner_iact_nonsym_bh_swallow()

Added diffusion reset in hydro when doing feedback and updated function signatures

See merge request !852

Lock the space before attempting to remove a particle from the system. This should prevent duplicated removals.

Implement the reduction of the accretion rate of the BHs based on the local viscuous time-scale in the EAGLE model.

Blackholes swallowing - Gas case

See merge request !845

Remove duplicate gpart communication

See merge request !848

Fixes #594

Closes #594

See merge request !849