compute_fwhm() throws exception on images with NaN

[JuanCab]

In essence, it would be nice if the photometry code in single_image_photometry() could use NaN to flag saturated pixels, as the photometry routines all know to ignore NaN and process the images correctly. The old photutils_stellar_photometry() function used this approach. Unfortunately, when calling compute_fwhm() an exception is thrown because NaN are not allowed in input data by the LevMarLSQFitter() function.

[mwcraig]

It would be worth checking to see whether there is a fwhm calculator in photutils these days, but otherwise I'm fine with using NaN.

photutils understands masks too, so that is another option if photutils has fwhm again.

[JuanCab]

Fixed in PR #145.