Add another communication sub-type for the gparts. Send the gparts when these tasks get enqueued.

This follows suit with the addition of task sub-types for the communications. I have added one to ship the gparts between nodes. 

This also allows to send/recv the gparts instead of the parts at enqueuing time. Note that the gpart communications are currently unused since I never pushed the support for gravity over MPI to the master. 

It also allows to only loop over the relevant bits we received and not over the whole particle zoo to construct the time information on the receiving side. 

This merges into the `new_timeline_mpi` branch and not in `master` as it is yet another MPI bug/feature fix.

See merge request !305

Revert "Force a sort of the particles at every time-step for active cells and at reception of data."

This reverts commit 078d450f

No need to check for received unactive cells for the rho communication sub-type. They can't happen any more.

Conflicts:
	src/task.c

Post repartition we can have cells if a bad state if they have previously been inactive on a rank (so they hold particles, but the cell data do not match)

Also check that all particles are in the correct cell at all levels when checking for h_max and dx_max

New time line

Changes include:
 - Time line is now based on a `long long` rather than `int` for more flexibility.
 - Particles only carry a `char` to indicate their time bin rather than two `int` for their start and end of time-step.
 - The kick task is not split in kick1, kick2 and timestep.
 - When compiling with the debugging checks, we verify that the particles have been properly drifted and kicked before any interaction.

See merge request !301