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.

Lots of refactoring to tidyup code reuse

add a parameter for space_subsize_self_grav, gravity self-interactions on cells with more than this number of particles will be split up.

This reverts merge request !375

Removed the now unused space_maxcount global variable. Documented the new YAML parameters in the example parameter file.