GDNUM

     GDNUM allows the user to select grids by number in the following ways:

	GDNUM = range        
 		The first and last grid numbers in the range are separated 
 		by a -.  If an increment is included, it will be ignored.
   
	GDNUM = list             
 		The grid numbers in the list are separated using semicolons.
   
	GDNUM = ALL                 
 		All the grids in the file will be included.
     
	GDNUM = LIST                 
 		All the grids in the file will be listed.  The user will 
 		be prompted to select the grids to be included.