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.