Noise term in extract spectra does not always include the sky noise

[ctheissen]

In image_lib.py:extract_spectra, I noticed that for some reason the 'k' term (line 215) is sometimes set to zero even though both the numerator and denominator have integer or float values. A quick fix for this is to change the numerator and denominator to float(x), where 'x' is either the numerator or denominator. I have no idea if this is a python version issue, but it's a very strange problem.