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.