@Override
  public IMemMNode addChild(final String name, final IMemMNode 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/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/mem/mnode/basic/BasicMNode.java