ENH: improvements for managing iso-surfaces

- generic isoSurfaceBase. Provides simpler cell-cut detection and
  various functions that can be used for iso-surfaces or when
  preparing prefiltered input for iso-surfaces.

- rudimentary runtime selection

ENH: isoSurface Cell/Topo uses the isoSurfaceBase infrastructure

- simpler cell cut detection, common routines
- ensure that tetMatcher is only called once per cell

ENH: use indirect patch during edge erosion

- lower overhead, allows backtracking (future) if needed