WIP - New Feature: Curvature field in FITS Viewer

Building up on the ability of the FITS Viewer to mark detected stars, this MR provides a picture overlay showing the curvature of the field of view.

The feature can be used for rough collimation and tilt adjustment using star fields. It is, however, not designed for scientific activities (maybe later?).

It uses the SEP algorithm calculations to extract the orientation and eccentricity of detected sources, and converts that sample cloud into a smoothed vector field, as shown in the picture below:

This picture of the Eyes region illustrates the strong coma on my Orion ED80T. The optical center is off (to the top right), and the back-focus distance does not seem correct (elongated stars on the left).

The color of the field is green when the eccentricity of detected sources is lower than 0.3, and red when it is higher than 0.8.

The feature is available from the FITS Viewer menu "View", item "Show Curvature". The computation is currently done each time the frame is updated, but is relatively quick. The resolution of the field is hardcoded at 30x30.

As the feature works best with a relatively high number of samples (but only stars obviously), an option was added to KStars to allow configuration of the maximum source count after which the SEP (and the Centroid) algorithms will stop searching. The option is available in the FITS Viewer options.