SUSTACK - stack adjacent traces having the same key header word

     sustack <stdin >stdout [Optional parameters]		

 Required parameters:						
 	none							

 Optional parameters: 						
 	key=cdp		header key word to stack on		
 	normpow=1.0	each sample is divided by the		
			normpow'th number of non-zero values	
			stacked (normpow=0 selects no division)	
 	verbose=0	verbose = 1 echos information		

 Note:	The offset field is set to zero on the output traces.	
 	Sushw can be used afterwards if this is not acceptable.	

 Credits:
	SEP: Einar Kjartansson
	CWP: Jack K. Cohen, Dave Hale

 Note:
	The "valxxx" subroutines are in su/lib/valpkge.c.  In particular,
      "valcmp" shares the annoying attribute of "strcmp" that
		if (valcmp(type, val, valnew) {
			...
		}
	will be performed when val and valnew are different.

 Trace header fields accessed: ns
 Trace header fields modified: nhs, tracl, offset