ENH: zone improvements

- retain group information when copying zones
- support construct empty (add details later)
- improve consistency for zone and boundaryMesh construction

- support front/back/both selection for faceZoneToCell

STYLE: prefer faceZone patch() method instead of operator()

STYLE: use std::unique_ptr instead of manual pointer management

- for zones and core patch types.
  Easier data management, allows default destructors (for example)