Cellular shutdown method

This method will properly and safely shut down the cellular modem.

cellular.shutdown(reset=[reset])

Where reset can be True or False.

If reset is set to True, the XBee Cellular will be rebooted after the cellular modem has been shut down.

If reset is set to False, the XBee Cellular will not be rebooted, but the cellular modem will have been shut down.

If False, you would typically use a machine.reset() after this command to emulate the reset=True option.