Allow for the visualization of small graphs using Graphviz

[angelip2303]

It would be cool to support an export format in Graphviz.

[labra]

Indeed, this functionality is a very nice feature to have.

For simple graphs, we can get inspiration from the implementation that we already did in Scala and it was quite easy to do. In that implementation, we defined an intermediate data structure called SGraph (for simple graph) and we converted RDF graphs to SGraph, and those SGraphs could be easily serialized to dot format.

This tools is used by rdfshape