Compute simple image statistics in a FITS image sub-region.

% clip_imstat.sh --help 

Usage: clip_imstat.sh a.fits 50 100 100 300
arg1 - input FITS image 1
arg2 - min i of image sub-region 
arg3 - max i of image sub-region 
arg4 - min j of image sub-region 
arg5 - max j of image sub-region