Added a higher-order base class

[nwlandry]

This PR does the following:

• Adds a HigherOrderNetwork base class for all classes to inherit from. Currently implements a constructor and some basic functionality.
• Now Hypergraph, SimplicialComplex, and DiHypergraph all inherit from HigherOrderNetwork. SimplicialComplex now no longer inherits from Hypergraph.
• Methods with warnings (e.g., add_edge) in SimplicialComplex have been removed.
• Renames the following. These are NOT breaking changes as they are all internal to XGI:
  • Renames _hypergraph to _net_attr
  • Renames _hypergraph_attr_dict_factory to _net_attr_dict_factory
  • Renames _hyperedge_dict_factory to _edge_dict_factory
• Up-versions pylint.