Accept non-hyperrectangular sets for the discretization with zonotopes

[mforets]

In #528 we added the option set_operations with default value "lazy". If set_operations is "zonotope", it uses concrete operations between zonotopes, and needs to convert to zonotope. Sets such as Ball2 is not allowed.

Proposal: if convert(Zonotope, X0) is not available, call overapproximate(X0, Zonotope) or (if it is not defined), just convert(Zonotope, overapproximate(X0, Hyperrectangle)).

[schillic]

I changed my mind here.
It does not make sense to use zonotopes if the sets in the input are not given as zonotopic sets. Since there is (probably) no generic overapproximation method for zonotopes, unexpected things can happen.
I would say that the user should take care of the conversion in advance. Hence I would say that we can keep the existing converts.

[mforets]

Alright, agreed.