Simplifations of sspz

[mforets]

The example added here highlights that we should have a simplification function such that:

• Zero columns in G can be eliminated.
• Zero columns in E can be associated with the center for the corresponding generator.
• Repeated columns in E can be grouped together by summing the corresponding generators.

(This simplification is exact btw)

cc @lucaferranti

[lucaferranti]

good catch, should we do this simplification always automatically in the constructor or should there be a function that must be explicitally called if the user wants to simplify, do you have something similar for other sets?

[mforets]

Maybe we can start by just writing a function remove_redundant_generators (there is one with the same name already for Zonotopes). Instead of applying it in the constructor, I think I'd rather use it in quadratic_map and other places that may produce redundant generators.