Mesh.ToleranceReferenceElement to control the tolerance in the reference space
- new PViewData::search*Closest to return the value at the closest node if an exact
  match does not exist
- rationalize the view::probe() API:
   * the "tolerance" is now the maximum distance allowed for a match ("distanceMax"),
     which can be 0 (exact match), positive or negative (always return the closest
     value if an exact match does not exist)
   * return the distance to the match
- remove extra #ifdef _OPENMP everywhere