Sensors.poll will eventually fail when using sample data

[tonyroberts]

Senors.poll will fail when it advances past the end of the sample data with an IndexError.

Instead it might be better to loop back to the beginning (sensors.py, line 233)

       def poll(self):
            """Pulls data from the pressure and flow sensors"""
            datum = tuple((self._fake_data.pressures[self._data_index],
                           self._fake_data.flow_rates[self._data_index])
                          for _ in range(NUMBER_OF_PATIENTS))
            self._data_index = (self._data_index + 1) % len(self._fake_data)
            return datum

[mmroden]

Great idea, would love to see this in a PR :)