#1270 - AbstractZonotope interface

[schillic]

Closes #1270.

I renamed generators to genmat (because it returned the matrix; I found it more natural that generators becomes an iterator over the generators).

Task list for a follow-up issue:

• do not create zero generators in LineSegment (check identity of vertices) and AbstractHyperrectangle (check for zero entries in radius_hyperrectangle) EDIT: I added it here to have a more consistent state
• generalize Zonotope methods to AbstractZonotope (should be possible in most places)

[mforets]

LGTM.

I left some comment regarding improving the docs, and a possible change in the way HyperrectangleGeneratorInterface is implemented.