ENH: additional filtering for distance surface (#1950)

- adds topology-based segmentation of the surfaces generated with
  distance surfaces. This can occur when the surface terminates
  close to a thin wall gap in the mesh; resulting in a cuts that
  extend into the next region.

  The cutting algorithm does not normally distinguish between these
  types of "ragged" cuts, and legitimate ones (eg, cutting multiple
  pipes). The additional segmentation controls provide for two common
  scenarios:

  largestRegion (pre-filter):
  - The cut cells are checked for topological connectivity and the
    region with the most number of cut cells is retained.
    This handles the "ragged" edge problem.

  nearestPoints (pre-filter):
  - The cut cells split into regions, the regions closest to the
    user-defined points are retained.
    Uses maxDistance for control.

  proximity (post-filter):
  - Checks the resulting faces against the original search surface
    and rejects faces with an absolute distance that is greater
    than (relProximity * normal distance).
    This traps the ragged overhanging cut faces that do not correspond
    to the originating surface normals.
    Uses relProximity for control.

ENH: restructure distance surface geometric filtering

- prefilter cells, which can be used to adjust the distance
  calculation in the far field to the real distance
  (not the normal distance).

  This can also be used to artificially sharpen the transition
  between near/far regions, if required in the future.