Synopsis
		sfseekwin < in.rsf > out.rsf whence=sf_seek_set nseek=10 nread=10

Works like sfwindow in 1-D. Extracts consecutive sequence of values in N-d.
The following commands should output the integer sequence from 10 to 19:
sfmath n1=20 o1=0 d1=1 type=float output=x1 > junk.rsf
Cannot take input from a pipe because stdin cannot be seeked through

Parameters
		nread=10   nseek=10   whence=sf_seek_set