Black holes mergers

See merge request !863

Fixes a bunch of warnings triggered by GCC 9.1

Closes #600

See merge request !867

Fix diffusion velocity bug

See merge request !865

Added nIFTy cluster example

See merge request !864

Also apply the distance criterion when checking for BH mergers. Read the correct value of G from the physical constant structure.

Improve the hydro task splitting logic to take into account the star content of cells as well.

See merge request !851

Ignore inhibited particles in timestep

See merge request !856

Fixed typo in the hydro-only task splitting criterion. Need to consider both ci and cj's part counts.

Split the star formation task into two parts

See merge request !844

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.