Refactor and improve structured node generation

[wstrinz]

allowing individual datapoint values to be rdf nodes with their own structured information (as opposed to simple literals) would significantly increase the expressive power of the gem's tools, and make compatibility with ontologies such as sio

A simple way to do this would be to use blank nodes, however this introduces problems in dataset sharing and interoperability, and is generally considered "bad style" in the Semantic Web community.

The alternative is generating unique URIs yourself. This should be possible using the dataset name and namespace, the label of the observation, the position of the value within the observation, and the depth of the structured node. A basic implementation is in the gem right now, but still has a few kinks and only works one node deep. For an example, see this gist

A proper implementation should interpret arrays in its input as nodes, and generate uris and rdf for them. Whatever method is involved should be recursive or have some other way to allow nodes of arbitrary depth as well.