Implement vertices_list for HPolytope

[schillic]

We have HPolytope <: AbstractPolytope, but this interface requires an implementation of vertices_list. Consequently, some operations break.

There is a bug in the interface checking function, hence we did not detect that this function is missing.

Actually, if we have vertices_list, we might as well add VPolytope because we get tovrep for free (we can leave tohrep open for now).

[mforets]

yes; this is an instance to use the polyhedra manipulation library.