- the surfMesh classes where originally designed with limited
  (protected) access to the underlying components. This is to
  avoid unintentional direct changes, since these can quickly
  lead to inconsistencies with the topology addressing etc.

  However, if we wish to efficiently adjust surfaces, it is useful
  to modify the components directly.
  The compromise is to provide 'xfer' methods:
    - xferFaces()
    - xferPoints()
    - xferZones()
  These transfer the contents to an Xfer container for reuse, while
  also resetting the topology addressing.  To apply the changes,
  the reset() method is used.