Creates a new, cropped version of an existing flow direction grid
handle_crp = CropFlowDir ( handle_src , CropWindow [, name] )
integer: handle on the flow direction grid to be cropped
vector of length 4: first row, first column, last row, last column
(optional) character string
integer: handle on the newly created, cropped flow direction grid
Creates a new, cropped version of an existing flow direction grid. Returns a handle on the new flow direction grid.