Report memory use of SWIFT

Fixes #414 (closed)

Adds a simple mechanism for getting reports of the main memory use in SWIFT. When not chosen the calls become no-ops. Each report is formatted like:

[0002] [00107.8] :memuse: 0:parts engine.c:engine_exchange_strays:1806 2052274

Where :memuse: is a parsing label for extracting the records, 0:parts shows that this is step 0 and a report of the memory used by parts, the next field is where the report originates and the final number the number of KBs in use.

These can be processed to give sums of each report per step by the process_memuse.py script.

Note that we report particle use, whether or not a reallocation has occurred during rebuild/repartition steps and the special label :step is used to report the memory in use by the process (i.e. resident set size) so that the fractions are clear and how much memory in actually used.

Changes to consider:

• write output to a file not stdout.