Move vertex facet graph to combinatorial polyhedron

[gh-kliem]

We move the method vertex_facet_graph from Polyhedron_base to CombinatorialPolyhedron.

Along the way we fix an bug, namely that vertex_facet_graph(labels=False) has the edges the wrong way::

sage: P = polytopes.cube()
sage: P.vertex_facet_graph().is_isomorphic(P.vertex_facet_graph(False))
False
sage: P.vertex_facet_graph().is_isomorphic(P.vertex_facet_graph(False).reverse())
True

[gh-kliem]

New commits:

​630b8a8	add vertex_facet_graph to combinatorial polyhedron
​6dbf267	use combinatorial polyhedron to obtain the vertex facet graph

[jipilab]

There is a space missing in the inputs.

Then, I would just swap the two examples to show the default behavior first.

Could you make this sentence more clear:

If ``names`` is ``True`` but no names are provided,

I would suggest to put If names is True (the default) but no names....

[gh-kliem]

New commits:

​9a62a52	add vertex_facet_graph to combinatorial polyhedron
​1e10ebd	use combinatorial polyhedron to obtain the vertex facet graph
​7bae9dc	improved doc

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​d4679c7

removed unused import

[gh-kliem]

I'll just put it back to needs work until this is done.

Replying to jipilab:

There is a space missing in the inputs.

Then, I would just swap the two examples to show the default behavior first.

Could you make this sentence more clear:

If ``names`` is ``True`` but no names are provided,

I would suggest to put If names is True (the default) but no names....

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​a36a211

swapped examples

[mkoeppe]

Batch modifying tickets that will likely not be ready for 9.1, based on a review of the ticket title, branch/review status, and last modification date.

[jipilab]

LGTM.