Inconsistencies in `__getitem__` implementation across complex types

[ffl096]

Similar to #266, the __getitem__ method is not implemented consistently across different complex types:

• For simplicial complexes, SC[simplex] returns the user-defined attributes associated with that simplex.
• For cell complexes, CC[cell] returns the neighbours of that cell.
• For coloured hypergraphs (and combinatorial complexes), CHG[node] returns the attributes of that node, but it does not work for hyperedges.
• For path complexes, PC[path] returns the attributes associated with that path.

[ffl096]

For context, NetworkX works as follows:

• G[node] gives a view of all neighbouring nodes.
• G[edge] is invalid.
• G[node][neighbour] gives access to the edge attributes of the edge between node and neighbour.
• G.nodes[node] and G.edges[edge] gives access to the attributes of node and edge, respectively.

That being said, I am not sure we want to mirror that API exactly. For example, SC[node][neighbour] is ill-defined, as there can be multiple connections between two nodes and it is not clear what attributes to return.