[Refactor] Refactor the graph classes to using the `CoreGraph`.

[daehyun99]

Sub-issue 3 of #2376

I will develop based on the meeting contents and the discussion in #2376.

Tasks:

1. Make DAG, PDAG, ADMG, and MAG inherit _CoreGraph.
2. Add SUPPORTED_EDGE_TYPES in each of these classes. For DAG: {'->', '<-'}, for PDAG: {'--', '->', '<-'}, for ADMG: {'->', '<->', '<-'}, for MAG: {'->', '<-', '--', '<->'}
3. Remove the duplicate methods such as add_node, add_edges, add_edges_from from these classes and let the _CoreGraph handle these methods.
4. Make type='->' as the default for add_edge and add_edges_from methods.
5. Replace the AncestralBase class to the _CoreGraph entirely.