NEW FEATURE - Star Roundness exposure rejection during stacking

[Paul Shears]

Enhance the FWHM filter to include measurement of the roundness of key stars during the stacking process. The idea is to detect elongated stars which are due to mount hiccups during the exposure and prevent these from being stacked.

One complication that may arise is that if the image suffers from significant aberrations, e.g. due to aggressive focal reduction, then the star shapes in the outer parts of the field of view will exhibit some eccentricity (comet or egg shapes) and would probably skew the overall value of your metric of mean roundness across the field. Maybe just concentrating on stars in the central part of the FOV would get around this potential issue.