ENH: revert regionSplit to older algorithm (issue #805)
- the algorithm was last used in OpenFOAM-2.4, after which it was replaced with a FaceCellWave version. Whereas the original (2.4.x) version exhibited performance degradation on very large meshes (with explicit constraints), the FaceCellWave version exhibited performance issues with large numbers of blocked faces. With large numbers of blocked faces, the FaceCellWave regionSplit could take between 10 to 100 times longer due to the slow propagation speed through blocked faces. The 2.4 regionSplit has been revamped to avoid local memory allocations, which appears to have been the source of the original performance issues on large meshes. For additional performance, intermediate renumbering is also avoided during the consolidation of regions over processor domains.