Merge branch 'send_dx_max_part' into 'master'
Also send dx_max_part and dx_max_gpart alongside ti_end_min at the end of a time-step when running with MPI Implements the solution to #367. Instead of just sending the time-step we send the distances moved by any part and gpart in that cell. I have also done some cleaning up around the packing and unpacking and improved the documentation. See merge request !434