Create an NMatrix transformer and writer

[wstrinz]

NMatrix is a linear algebra library written in Ruby, and one of the core gems of SciRuby. A PubliSci::Writers class that takes Data Cube RDF as input and creates NMatrix objects, or a Publisci::Readers that does the opposite, would help integrate this gem with the rest of SciRuby, and open up interesting possibilities for sharing and discovering datasets.

For examples of Writer classes, see the csv and arff writers. A transformer that takes matricies as inputs (from R) is also available for reference.