as.SpatialGrid,prevR-method: Create a spatial grid from an object of class prevR.
Description
This function generates a spatial rectangular grid from the slot boundary
of an object of class prevR; function used in particular
by the methods kde, krige
and idw.
This function generates a spatial rectangular grid, each cell being a square
of side cell.size. If cell.size is not defined, side of cells will be
calculated as the longuest side of the slot boundary of object divided
by nb.cells.