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.
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• I have created tests covering the changes.
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[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]

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