Also send dx_max_part and dx_max_gpart alongside ti_end_min at the end of a time-step when running with MPI.

Lots of refactoring to tidyup code reuse

Previously we had a max of 96, much smaller numbers will work depending on the job

Create the gravity pair tasks over the periodic boundary conditions. Make the long-range gravity task loop over the cells even over the periodic boundaries.

This reverts merge request !375

remove tests for undrifted particles in send/recv as not all sent/recvd particles are drifted, only those that are needed. note that undrifted particles will still be caught in the interaction functions.

Create the gravity pair tasks over the periodic boundary conditions. Make the long-range gravity task loop over the cells even over the periodic boundaries.

Create the gravity pair tasks over the periodic boundary conditions. Make the long-range gravity task loop over the cells even over the periodic boundaries.

Create the gravity pair tasks over the periodic boundary conditions. Make the long-range gravity task loop over the cells even over the periodic boundaries.

Modified the splitting rule for self tasks to be less conservative. Added proper documentation of the splitting conditions.

Also create functions to decide when to rebuild and when to split a task. Update the rebuild and split condition to correctly use kernel_gamma.

change sort task flags to be zero by default and only switch on those that are needed, avoids pre-emptively computing all the sorts for a cell since they may never be needed between two sort cleanups.

replace 'wait' with 'implicit' in scheduler_addtask, as we weren't using the former and could use the latter.