- helps avoid the creation of small face cuts (near corners, edges)
  that result in zero-size faces on output.

CONFIG: make default iso-surface topo regularisation less aggressive

- The full (diagcell) regularisation no longer includes cleaning of
  non-manifold surfaces by removing open edges.
  This can be selected by the 'clean' regularisation option instead.
  ie, 'clean' = 'full' + erode open edges

ENH: additional debug modes for iso-surface topo

- with (debug & 8) dumps out a VTK file of the tets to be cut and the
  calculated open edges.