ENH: improvements in the surface sampling infrastructure

- improvement documentation for surface sampling.

- can now specify alternative sampling scheme for obtaining the
  face values instead of just using the "cell" value. For example,

      sampleScheme    cellPoint;

  This can be useful for cases when the surface is close to a boundary
  cell and there are large gradients in the sampled field.

- distanceSurface now handles non-closed surfaces more robustly.
  Unknown regions (not inside or outside) are marked internally and
  excluded from consideration. This allows use of 'signed' surfaces
  where not previously possible.