Add deformation cones and checking for regularity for Point Configurations and normal fans of Polyhedra

[jplab]

In this pull request, we add the method deformation_cone (to point configurations and polyhedron) and is_polytopal (to fans).

This is related to the Kahler cone of Toric Varieties, but has some subtle differences to make it work as it should in the discrete geometry context. Therefore, it has a separate implementation.

TODO: In the future, perhaps it could be fusioned.

sage: tc = Polyhedron([(1, -1), (1/3, 1), (1, 1/3), (-1, 1), (-1, -1)])
sage: dc = tc.deformation_cone()
sage: dc.an_element()
(2, 1, 1, 0, 0)
sage: [_.A() for _ in tc.Hrepresentation()]
[(1, 0), (0, 1), (0, -1), (-3, -3), (-1, 0)]
sage: P = Polyhedron(rays=[(1, 0, 2), (0, 1, 1), (0, -1, 1), (-3, -3, 0), (-1, 0, 0)])
sage: P.rays()
(A ray in the direction (-1, -1, 0),
 A ray in the direction (-1, 0, 0),
 A ray in the direction (0, -1, 1),
 A ray in the direction (0, 1, 1),
 A ray in the direction (1, 0, 2))

sage: py = Polyhedron([(0, -1, -1), (0, -1, 1), (0, 1, -1), (0, 1, 1), (1, 0, 0)])
sage: dc_py = py.deformation_cone(); dc_py
A 4-dimensional polyhedron in QQ^5 defined as the convex hull of 1 vertex, 1 ray, 3 lines
sage: [ineq.b() for ineq in py.Hrepresentation()]
[0, 1, 1, 1, 1]
sage: r = dc_py.rays()[0]
sage: l1,l2,l3 = dc_py.lines()
sage: r.vector()-l1.vector()/2-l2.vector()-l3.vector()/2
(0, 1, 1, 1, 1)

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

[jplab]

I still need to add the method is_regular and is_projective... Will do very soon.

[seblabbe]

I made some changes to the code of is_regular. It is available in the branch of the same name in my github.

[github-actions]

Documentation preview for this PR (built with commit 2a9328a; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[fchapoton]

looks good. Let us wait for the continuous integration