ENH: regionSplit: improve algorithm order
The old version of regionSplit would hand out regions one by one. This is a big problem when there are lots of regions - the extreme being in the decompositionMethods, where it is used to cluster cells and most clusters being only one cell. This rewrite uses a mesh wave to determine disconnected regions in one go. This produced non-compact numbering which is then compacted in a second phase. On a 14M cell case with cyclic constraints this reduced decompose time from 40 mins down to 5.