set arbitrary mode

def cmd_tracker_mode(self, args):
        '''set arbitrary mode'''
        connection = self.find_connection()
        if not connection:
            print("No antenna tracker found")
            return
        mode_mapping = connection.mode_mapping()
        if mode_mapping is None:
            print('No mode mapping available')
            return
        if len(args) != 1:
            print('Available modes: ', mode_mapping.keys())
            return
        mode = args[0].upper()
        if mode not in mode_mapping:
            print('Unknown mode %s: ' % mode)
            return
        connection.set_mode(mode_mapping[mode])