Replace manual generation of turtle strings with ruby-rdf where possible

[wstrinz]

Any part of the core data_cube.rb module that can be rewritten to use ruby-rdf rather than string manipulation would help with future development of the gem (as long as the tests still pass and performance doesn't take too big a hit)

Early on I made the decision to generate the rdf turtle output for the gem using mostly string manipulation. Ruby makes this easy and pretty, but ultimately it is a less flexible, testable, and reliable way than using good object oriented design to build up an output graph.

In retrospect, I should have used the excellent ruby-rdf gem more often for building statements, Converting ruby literals to RDF literals, and serializing or loading the output.