Uses '1' as we could possibly have a second or third requirement for collections...

Merge two MPI_Allreduce calls into one MPI_Allgather in an attempt to minimize the global synchronization overheads

cputimes output file to avoid an MPI call each step. General tidyups
to remove unneccesary values and to keep APIs clean.

Stop repartitioning in the step after a repartition, that makes no sense and is unlikely to give interesting CPU times. Also remove some development output from timestep file

CPU time fraction. Also allow the balance test to be done if fewer
than the complete number of particles interact. Finally capture the
CPU times to a file so that the balance can be assessed. This is
only done when task timings are also enabled.

Compute the total mass in the system and add a variable to the gpart to record how much mass they interacted with via self-gravity.

Added function to drift the multipole of a given cell without updating its progenies.