Class Circle is defined in circles.web.
It is derived from Ellipse, using public derivation.
Since Circle is just a special kind of Ellipse, there is
often no need to define special functions for Circles.
Currently, Circle inherits the transformation functions and
operator*=(const Transform&) from Ellipse. Consequently,
the data member radius, described below,
is not recalculated, when transformations
are performed on a Circle. I plan to change this soon!