Create the M-M gravity task one level higher. The task itself recurses on its progenies. We can run with a rebuild every time-step. More work required on the unskip.

Prevent users from generating gas from the ICs if they are running without the hydrodynamics policy.

Parallel mesh assignment and simplification of M2L kernel

See merge request !604

Correct debugging check for the size of the collected time-step. A step of size 0 is acceptable on the last step.

Only collect time-steps for hydro and gravity for the cells that have related…

Closes #455

See merge request !605

Use a dedicated MPI type for the pcells. Also use the m-pole MPI type for the communications instead of MPI_BYTE.

Correctly move the reading pointers for the various chunks in the parallel HDF5 i/o module. Increase the hdf5 chunk size to 1MB

Use temporary local copies of the D_xxx terms in the M2L_apply function to help the compiler optimize quantities.

Use the threadpool and atomic operations to parallelize the assignment of gparts to the density mesh.