Indicate origin of tolerance ellipses in diagnostic plot

[aalfons]

Suggestion by Patrick: add argument origin (possible values TRUE or FALSE) to ellipse_plot() to indicate the origin of tolerance ellipses, for instance with light gray horizontal and vertical lines.

[aalfons]

This could quickly make the plot overloaded, as we can also have lines in some cases to illustrate the corresponding (partial) effect. I think it would be more useful if setup_ellipse_plot() would return the center of the tolerance ellipse in a component center. Then users can easily add those lines using geom_hline() and geom_vline().