step_info problem with non stationary systems response #564

Systems with non stationary response to step input, step_info could give wrong information:

import numpy as np              # NumPy library
import matplotlib.pyplot as plt
import control as ctrl  # Name: control Version: 0.8.4


sys2 = ctrl.tf(1,[1,1,0]) 
time,yout=ctrl.step_response(sys2)
plt.figure()
plt.plot(time,yout)
info=ctrl.step_info(sys2)

{'RiseTime': 19.0,
'SettlingTime': 24.5,
'SettlingMin': 21.500000000169212,
'SettlingMax': 23.750000000017877,
'Overshoot': 0.0,
'Undershoot': 0.0,
'Peak': 23.750000000017877,
'PeakTime': 24.75,
'SteadyStateValue': 23.750000000017877}

The step_info expected result for this system could be:

tupla with fields:

RiseTime : nan
RiseTimeInit : nan
SettlingTime : nan
SettlingMin : nan
SettlingMax : nan
Overshoot : nan
Undershoot : nan
Peak : inf
PeakTime : inf
SteadyStateValue : nan