@Override
  public IConfigMNode addChild(String name, IConfigMNode child) {
    return null;
  }
add a child to current mnode

@param name child's name
@param child child's node
@return the child of this node after addChild
iotdb-core/confignode/src/main/java/org/apache/iotdb/confignode/persistence/schema/mnode/basic/ConfigBasicMNode.java