Measurements done by means of PlanarFigure objects should be directly available from the PlanarFigure interface.

Because the individual PlanarFigure sub-classes can have quite different measurable features (such as length, circumference, radius, angles, area, ...), the interface should be generic enough to handle all of these. Each feature should be designated by (at least) the measured quantity, a name (e.g., "Length"), and a unit (e.g., "mm").