* [For CRAN release] Terminate code / doc for interp_Grid, including
  examples. Then remove the 'GrindInt' and 'GridIntCB' from the
  exported functions and make sure that their doc is no-longer
  visible.

* Define a S4 class for function evaluation on a grid? Could be useful
  for 'plot' and 'contour' methods (give only one argument instead of
  3).

* [DONE] Write benchmarks with the test functions.


