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.