Getter function for the generators of a Zonotope

[aa25desh]

Closes #1309.

[schillic]

You need to add the following:

• Add export generators at the top of Zonotope.jl. This makes the function available without explicitly importing it.
• Add the function to the documentation in docs/src/lib/representation.jl under ## Zonotope. Otherwise our build will fail.

Also, we try to have unit tests for everything in this package, so please add one to test/unit_Zonotope.jl.

[schillic]

The unit test here would be very silly, but still it's better to have it:

@test generators(Z) == Z.generators

for some zonotope Z.

[schillic]

Thanks. We want to fix some internals first, so let's wait with merging here. Maybe I can fix things tomorrow.

[schillic]

@aadesha: If you want, you can add your name here (in a new pull request).