[Rd] Suggestion: Dimension-sensitive attributes

[Bengoechea Bartolomé Enrique (SIES 73)]

> In the case the metadata are stored in a list, that interface enforces the building of a list.
> (I said to ignore implementation for now, but paradoxically this made me consider possible implementations).

Creating the list on the fly if it's not stored internally as a list should be cheap. For example, this is done with data frames, that store "dimnames" in two separate attributes, "names" and "row.names".