This module builds a Dlist containing a regular sampling with
$n$ points of a circle of radius $r$~:
$$P_k = \left(\begin{array}l r\cos(\frac{2\pi k}n)\\
r\sin(\frac{2\pi k}n)\end{array}\right), \qquad k=0,1..n.$$
The resulting curve is closed: its first and last points have
exactly the same coordinates.

