Feature topo set improvements (issue #1060)

- make topoSet set/unset methods virtual to allow overloading

- old 'DELETE' enum was easily confused with 'REMOVE', which removes
  the set, not the elements from the set.

- provide corresponding subtractSet() method

STYLE: HashSet set/unset instead of insert/erase methods in topoSetSource

- simplifies switching to/from bitSet storage

- add intermediate classes topoSetCellSource, topoSetFaceSource,
  topoSetPointSource and corresponding New() factories

- Support specification with "patches" and "patch" keywords
  (similar to zone selection). Keyword "name" for compatibility.

- "boxes" has precedence over "box" (similar to treatment of "zones"
  vs "zone")

- replaces "centre", using the same keyword as other objects
  (eg, plane, rotatedBox, coordinateSystem etc).

- uses the keywords 'zones' and 'zone' to avoid potential conflicts
  with a named topoSet action, but accepts 'name' for compatibility.