This is an adaptation of the projecting method for ellipse introduced by D. Eberly.
Internet publication: "Distance from a point to an ellipse in 2D" (2004) Geometric Tools, LLC, www.geometrictools.com
Book publication: "3D Game Engine Design", 2nd edition.
Morgan Kaufmann Publishers, San Francisco, CA, 2007.
(see Section 14.13.1)
Usage: [RSS, XYproj] = Residuals_parabola(XY,ParG)