return dictionary of column labels if available

def get_column_labels(command_name):
    '''return dictionary of column labels if available'''
    cmd = cmd_reverse_lookup(command_name)
    if cmd == 0:
        return {}
    labels = {}
    enum = mavutil.mavlink.enums['MAV_CMD'][cmd]
    for col in enum.param.keys():
        labels[col] = make_column_label(command_name, enum.param[col], "P%u" % col)
    return labels