SUBFILT - apply Butterworth bandpass filter 			

 subfilt <stdin >stdout [optional parameters]			

 Required parameters:						
 	if dt is not set in header, then dt is mandatory	

 Optional parameters: (nyquist calculated internally)		
 	zerophase=1		=0 for minimum phase filter 	
 	locut=1			=0 for no low cut filter 	
 	hicut=1			=0 for no high cut filter 	
 	fstoplo=0.10*(nyq)	freq(Hz) in low cut stop band	
 	astoplo=0.05		upper bound on amp at fstoplo 	
 	fpasslo=0.15*(nyq)	freq(Hz) in low cut pass band	
 	apasslo=0.95		lower bound on amp at fpasslo 	
 	fpasshi=0.40*(nyq)	freq(Hz) in high cut pass band	
 	apasshi=0.95		lower bound on amp at fpasshi 	
 	fstophi=0.55*(nyq)	freq(Hz) in high cut stop band	
 	astophi=0.05		upper bound on amp at fstophi 	
 	verbose=0		=1 for filter design info 	
 	dt = (from header)	time sampling interval (sec)	


 Credits:
	CWP: Dave for bf.c subs and test drivers
	CWP: Jack for su wrapper

 Caveat: zerophase will not do good if trace has a spike near
	   the end.  One could make a try at getting the "effective"
	   length of the causal filter, but padding the traces seems
	   painful in an already expensive algorithm.

 Trace header fields accessed: ns, dt, trid