This PR fixes the non-intuitive time-domain simulation behavior for discrete time systems identified in issue #239. In this version:

• The values of the output argument T always matches the times at which the corresponding outputs Y are obtained. This is consistent with the idea that you plot the outputs of a simulation function by using commands of the form

t, y = step_response(sys)
plot(t, y)

• If you pass an explicit set of sampling times for a discrete time system, the time increment must be an integer multiple of the system sampling time (with dt=True allowing any time increment). If not, a ValueError exception is raised.

• If you omit the sampling time, the value of T will be determined based on either the input signal (for forced_response() or the time constants coming from the system dynamics. In either case, the difference between time points will be equal to the sampling time of the system (or 1 if dt=True). (Note: net yet clear whether the time constant calculation is correct for discrete time systems; will create a new issue to track this.)

Unit tests are included to make sure all of the statements above are checked.