open usb port

    Args:
      port_num: port number on the Cambrionix unit

    Return:
      usb handle

def open_usb_handle(self, port_num):
    """open usb port

    Args:
      port_num: port number on the Cambrionix unit

    Return:
      usb handle
    """
    serial = self.get_usb_serial(port_num)
    return local_usb.LibUsbHandle.open(serial_number=serial)