Circle Fit (Pratt method)

Fits a circle to a set of data points on a plane; returns the circle center (a,b) and radius R.

This is a robust and accurate circle fit. It works well even if data
points are observed only within a small arc. This circle fit was proposed by V. Pratt in article "Direct least-squares fitting of algebraic surfaces", Computer Graphics, Vol. 21, pages 145-152 (1987). It is more stable than the simple Circle Fit by Kasa (file #5557).