Free sort indices and task arrays before repartitioning

Freeing the sort indices and tasks arrays before repartitioning makes that memory available so that we have more room for accepting particles from other nodes.

Part of #98 (closed).

Also adds a useful debug function that reports the current memory usage of the process tree (i.e. call from main thread to get all usage). Same numbers you get out of the top program.

Free the task arrays and cells are refactored into simple functions to cut down on cut and paste.

Added 1 commit:

• ea4268c2 - Also free the task arrays for more headroom before repartitioning.

Title changed from Free sort indices before repartitioning to Free sort indices and task arrays before repartitioning

mentioned in issue #98 (closed)

Added 2 commits:

• cdf04591 - Squash error about unused return value
• 888f5c2a - Also free the tasks in space_regrid before partitioning.

Updated to also free the tasks before a repartition caused by an increase in h. Wrapped up a lot of repeated code into some simple functions.

LGTM!

Reassigned to @pdraper

Status changed to merged

mentioned in commit e2650d1d

Thanks, I've accepted.