Implement get_point_position for Polygon (and not only ConvexPolygon)

[videlec]

The method ConvexPolygon.get_point_position returns a relative position of the point with respect to the polygon (eg in the interior, on an edge, at a vertex or outside). It does not exist for Polygon even though it does make sense for them. There should exist a general version for Polygon (probably going through the triangulation of the polygon).

It might also be desirable to promote Polygon to ConvexPolygon in the constructors when the input is indeed convex.

[saraedum]

It might also be desirable to promote Polygon to ConvexPolygon in the constructors when the input is indeed convex.

That's been fixed now.