ScalES-PPM

The repartition-by-swapping routines can benefit from the following improvements:

• repartition in parallel by pairing up lowest weight with highest weight part, second-lowest weight with second-highest weight part etc.
• sort the larger part weights (size n) to achieve O(log n) lookup

This issue is mostly for documenting the progress of an on-going implementation: the above is done for the variant for multi-process and 4 byte integer weights (mp_i4) the following are required for completion:

• mp_dp
• mt_i4
• mt_dp

Where the multi-threaded variants have yet to be written.