Generalized definition of plotting contours

[mlincett]

From the discussion of #25 , it seems we need a generalized way of defining the contours to plot when calling a plotting function.

The following scenarios should be supported:

• likelihood-based contours
  • Wilks's critical values;
  • arbitrary critical values (current systematic rescaling);
• circular contours with arbitrary radius
  • centered on the best-fit;
  • with arbitrary center direction;
• arbitrarily-shaped contours defined by the boundary coordinates.

Proper implementation of this will require a class / data model to represent the contour itself.