`dg::Map` needs `removeNode` and `removeEdge` function for consistency of lookup tables.

[sunglok]

Node and edge retrieval in dg::Map relies on lookup tables (built by std::map). If an user remove any node and edge directly from dg::Map::nodes and dg::Map::edges, it will cause inconsistency of data and their lookup tables. It is potentially dangerous.

Therefore, dg::Map needs removeNode and removeEdge which consider the lookup tables.