- an alternative to distanceSurface (with distance zero) that uses
  a cell cutting approach instead of an iso-surface.

- 'signed' input parameter only mandatory for distance > 0.
  A distance <= 0 is always signed and the input parameter is ignored.

- Use normal distance when distance == 0. This has no effect when
  the surface has no open edges, but improves on rounding issues
  around the zero crossing when the surface has open edges.

  This may still need future revisiting.

- avoid duplicate code by relocating cellZone selection and bounding box
  sub-selection into cuttingPlane and cuttingSurfaceBaseSelection.
  Allows reuse by inherited classes (sampledPlane, surfMeshSamplePlane).

- takes two general actions:
  1. orient edge in canonical direction (positive gradient) and detect
     any edge intersection.
  2. edge intersection alpha (0-1)

- refactor into a cuttingSurfaceBase intermediate class with the
  actions as templated parameters rather than function pointers. This
  allows the use of lambda functions with captures from the caller.

- The test condition

      [ -n "$cur" -a ... ]

  fails if $cur starts with '-le', which bash interprets as a further
  test op. Splitting the test condition solves the problem:

      [ -n "$cur" ] && [ ... ]

- Since the local edges are oriented according to the gradient,
  they can also be used to determine the correct face orientation.

This generalizes the algorithm for future reuse.

- simple check for unique and non-negative point labels

- replaces previous pointer conversion

- uses int, without enclosing brackets.

This is not the ultimate fix; it just avoids the loops in cases where
mesh 0 is the background.

mapping.  Fixes #983

Note: this removes the option to reconstruct by setting nDomains to 1.
      You now have to explicitly use -reconstruct to reconstruct.

- more consistent with dictionary method naming. The get<> or
  getList<> returns a value, doesn't read into a existing location.