grids.
* Now both inter{p|pp}.{old|new} print warnings and divert to inter{p|pp}.
* ACM 761 code updated to the revised version by Renka and Brown from 1998.
* Fixed two array overruns in ACM 761 code (work array size increased,
  thin triangle removing step cancelled if necessary).
* Thin triangle removing can now be omitted by paramater remove=FALSE
  (this is default for linear interpolation).