Prepare setting up polyhedron from both Vrep and Hrep for different backends

[gh-kliem]

#22701 implemented setting up a polyhedron from both Vrep and Hrep for the backend field.

There are plans to allow the same for polymake #26368 and with #28639 at hand we can do the same for normaliz.

So it makes sense to set up a general framework for initializing from Vrep and Hrep. This ticket does this (or rather moves what was done in #22701 accordingly).

If a backend implements the method _init_from_Vrepresentation_and_Hrepresentation, then the initialization of Polyhedron_base may use it if Vrep_minimal and Hrep_minimal is both set to true.

This option will not be exposed in the function Polyhedron, as a user should really know what they are doing.

Also the method _element_constructor_polyhedron of Polyhedra_base sets up the new polyhedron from both representations, if self.Element has the method _init_from_Vrepresentation_and_Hrepresentation.

[gh-kliem]

New commits:

​ffcc0bd

prepare setting up from both Vrep and Hrep in general

[git]

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

​2c9321e

removed unused import

[Erik Bray]

Ticket retargeted after milestone closed

[gh-kliem]

New commits:

​cb91aa5	prepare setting up from both Vrep and Hrep in general
​cc8765a	removed unused import
​561a629	fixed pyflakes warning

[Jean-Philippe Labbé]

Looks good to me! Thanks.

Sorry for the wait...