"look up" a face in the face lattice of a polyhedron

[gh-kliem]

We implement two methods that look up a face in the face lattice of a polyhedron:

• meet_of_Vrep -- the smallest face containing specified Vrepresentatives
• join_of_facets -- the largest face contained specified facets

This allows an easy answer for ​​https://ask.sagemath.org/question/34485/what-is-the-most-efficient-way-to-look-up-a-face-in-the-face-lattice-of-a-polyhedron/#50965

[gh-kliem]

Last 10 new commits:

​295039a	documentation
​d36da4a	coverage and small improvement
​2d0f0d9	method `reset` for the face iterator
​6d99a4c	typo
​5711f6b	method `ignore_subsets`
​f6633bd	method current and fix for reset
​e8c17c3	join_of_Vrep and meet_of_facets for face iterator
​b371a73	expose in combinatorial_polyhedron
​954b5c8	raise index error for index error
​521f9e0	expose in Polyhedron_base

[gh-kliem]

There are failing tests. I would wait until the dependices are taken care of to make things work again.