SUPGC   -   Programmed Gain Control--apply agc like function	
              but the same function to all traces preserving	
              relative amplitudes spatially.			
 Required parameter:						
 file=             name of input file				

 Optional parameters:						
 ntrscan=200       number of traces to scan for gain function	
 lwindow=1.0       length of time window in seconds		



 Author: John Anderson (visitor to CWP from Mobil)

 Trace header fields accessed: ns, dt