gr_id	IN:	GR interface identifier returned by GRstart
	n_images	OUT:	Number of raster images in the file
	n_file_attrs	OUT:	Number of global attributes in the file

	Purpose	Retrieves the number of raster images and the number of global attributes in the file.
	Return value	Returns SUCCEED (or 0) if successful and FAIL (or -1) otherwise.
	Description	GRfileinfo retrieves the number of raster images and the number of global attributes for the GR interface identified by the parameter gr_id, and stores them into the parameters n_images and n_file_attrs, respectively.
		The term "global attributes" refers to attributes that are assigned to the file instead of individual raster images. These attributes are created by GRsetattr with the object identifier parameter set to a GR interface identifier (gr_id) rather than a raster image identifier (ri_id).
		GRfileinfo is useful in finding the range of acceptable indices for GRselect calls.

	FORTRAN	integer function mgfinfo(gr_id, n_images, n_file_attrs)
		integer gr_id, n_images, n_file_attrs