Tidies up the restart option

Start reorganizing so we can restart, doesn't work yet

Correct wrapping of particles

See merge request !489

Better naming convention for the cell_split variables in the YAML file. New variable for the splitting of gravity pairs.

This reverts commit 10f04895.

When two particles are at the same position in ICs give more information about the particles involved.

Added a branching function for DOSELF_SUBSET. To call vectorised version when…

See merge request !488

Added a branching function for DOSELF_SUBSET. To call vectorised version when vectorisation is enabled.

Missing engine declaration when debugging checks enabled.

See merge request !487

Dopair subset vec

See merge request !480

Rework of Grackle

See merge request !484

Re-introduce the 'end_force' function as a task in order for the cooling function to be called at the right place in the time-step.

Align the runner structures on a memory bound.

Closes #395

See merge request !481