- remedy by performing the attach() action sequentially (as per
stitchMesh changes). This ensures that the current point addressing
is always used and avoids references to the already-merged points
(which is what causes the failure).
ENH: improve handling of empty patch removal
- only remove empty *merged* patches, but leave any other empty
patches untouched since they may intentional placeholders for other
parts of a workflow.
- remove any empty point/face zones created for patch merging
- constructor for empty cell/face/point Zones, with contents to be
transferred in later.
- ZoneMesh::operator(const word&) to return existing zone or a new empty one.
