Step response of continuous-time system.

    Parameters
    ----------
    system : an instance of the LTI class or a tuple of array_like
        describing the system.
        The following gives the number of elements in the tuple and
        the interpretation:

            * 1 (instance of `lti`)
            * 2 (num, den)
            * 3 (zeros, poles, gain)
            * 4 (A, B, C, D)

    X0 : array_like, optional
        Initial state-vector (default is zero).
    T : array_like, optional
        Time points (computed if not given).
    N : int, optional
        Number of time points to compute if `T` is not given.

    Returns
    -------
    T : 1D ndarray
        Output time points.
    yout : 1D ndarray
        Step response of system.

    See also
    --------
    scipy.signal.step2

    Notes
    -----
    If (num, den) is passed in for ``system``, coefficients for both the
    numerator and denominator should be specified in descending exponent
    order (e.g. ``s^2 + 3s + 5`` would be represented as ``[1, 3, 5]``).

    