lti.damp is implemented only for continuous time systems

The `damp` function in [lti.py](https://github.com/python-control/python-control/blob/33bebc190ecb3f63a7da5750710dd235bfd24cce/control/lti.py#L87) is only implemented for continuous time systems, which is not mentioned in the doc string and does not return an error for discrete systems. 

I will submit a PR to add support for discrete time systems. 
