Setters/Getters of Attributable

It's cool that a `Container` itself is *attributable*, but why are attributes not simply implemented as `Container` themselves as well?

I think similar to `h5py`'s [`AttributeManager`](http://docs.h5py.org/en/latest/high/dataset.html#creating-and-reading-empty-or-null-datasets-and-attributes), using our container both for data sets and attributes would make it very natural to interact with them. Or am I missing something?