change file time_string for windows compatibilty

base.py

@@ -466,7 +466,7 @@ if __name__ == "__main__":
 
     if True:
         # store results (only the values of limited buffers are written)
-        time_string = time.strftime("%Y-%m-%d__%H:%M:%S")
+        time_string = time.strftime("%Y_%m_%d__%H_%M_%S")
         sim.plot_sensor_timeseries(sensors)
         sim.write_sensor_timeseries(sensors, time_string)
         sim.make_video(floor, vehicle_1, floor_arrays, wheel_positions_1, time_string)
\ No newline at end of file