Feature/cardiac benchmark
This MR adds features for completing the cardiac electrophysiology benchmark problems and support to handle using a pre-partitioned mesh to avoid excessive memory requirements on the root node. Specifically, it adds the following features:
- Allow the mesh to be partitioned and then the solver exits (new option --part-only).
- Support in the mesh partitioner to split mesh into a specific number of partitions rather than being fixed at the size of the row communicator (i.e. a serial process can now be used to pre-partition the mesh).
- Allow a simulation to be started from the pre-partitioned *_xml file.
- Added a ThresholdMin filter to complement the ThresholdMax filter.
- Added a Benchmark filter in the Cardiac solver for outputting the activation and repolarisation times for comparing with established benchmark problems.