This PR gets rid of warning messages introduced in PR #662:

• Since rootlocus_pid_designer leaves either r or d disconnected, the interconnect function was generating warnings (76 of them...). This was fixed by using check_unused=False.
• Calling rootlocus_pid_designer with plot=True seems to generate a Bode plot that is -1 and this creates a warning:

control/ctrlutil.py:75: RuntimeWarning: invalid value encountered in remainder
  dangle_desired = (dangle + period/2.) % period - period/2.

For now this latter problem is fixed by skipping this test.

@sawyerbfuller I'm not quite sure how to fix this error. If I try running the test by hand using

ct.rootlocus_pid_designer(syscont, input_signal='r', Kp0=0.01,
    derivative_in_feedback_path=True, gain='P')

then I get the following plot: