implement random_element for EquiangularPolygons

[videlec]

When the angles correspond to a convex polygon (ie angle < pi) everything is easy: any non-negative combination of the rays give rise to a non-intersecting polygon. For n=4, one can work out the geometry easily, but starting from n=5 there are some delicate examples such as E = EquiangularPolygons(1,1,1,2,8). This latter example either fails with

• ValueError: edge [...] and edge [...] intersects or
• ValueError: the vertices are in clockwise order (which looks like a bug)

The article https://arxiv.org/pdf/2008.10192.pdf might be helpful.
See also #31