Need to add an interface to a mesh partitioner for MPI runs

[Slaedr]

Currently, for getting multi-process performance, the cells in the mesh need to pre-ordered by partition index. An interface with Scotch is envisaged to remove this requirement and partition the mesh as a preprocessing step.

Scotch is a good option because the move to PT-Scotch will be easier when the time comes. It appears that ParMetis is not free/open-source.

[Slaedr]

A Scotch interface has been added in the MPI branch, but it currently does not work. Perhaps the issue is with subdomain restriction, and the trivial partition only happens to work.