Moved restart_write into engine_step and trigger after drift_all

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

This reverts commit 10f04895.

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.

Correctly compute the separate time-bins for hydro and gravity at the bottom of the tree in space_split_recursive()

Separate the time-step information for hydro and gravity tasks into two different set of variables for each cell.

Fixed a moronic bug. Unskipping MPI sends and recvs for gravity should not be done *only* for local pairs. Obviously....

Instead of listing local cells only, list cells that have a tasks associated with them on the local node.

All-reduce all the top-level multipoles after a rebuild() to make sure we have all information in hands to construct the tasks.

Force compiler to report shadowed variables. Removed all shadowed variables. Added the function scheduler_activate_send() to simplify the unskipping of MPI sends.