Add ability to use RANSAC for robust registration with many points

Currently, affinder uses least squares to estimate the transformation between all user selected keypoints. However, it might not always be the best choice, as some user selected points may be more accurately placed than others (due e.g. to the sharpness of image features in different parts of the image). Additionally, users may want to use pre-computed points (see #11, for example), which could come from automated methods). Using RANSAC to match points will increase the reliability of found alignments.

Start by locating where affinder currently estimates the affine transformation with least squares, then review issue #11 for the pre-computed point context. Done means users can use RANSAC for robust registration when correspondences include inaccurate or automatically generated points, while the existing alignment behavior remains understood.

Written by the indexing model from the issue text.