Remove support for python-control
Remove internal class_utils.zpk (redundant).
Revert "Remove support for python-control"

This reverts commit 8458d0850692965d7a0e3d57f2a60c1bedd2af16.

It was here erroneously assumed that scipy LTI objects have no `inputs`
and `outputs` attributes. That's wrong. Revert this commit.