11.2.4 grid_set_dirty: flag cached grid as invalid

      subroutine grid_set_dirty(record)
      implicit double precision (a-h,o-z)

Mark the grid as dirty, i.e., any cached information should be discarded. Typically, this will be called after changing any parameters, or on changing geometry. If record is negative, the grid is marked as 'very dirty': parameters changed to the extent that grid_generate will need to be rerun