FIX: Repaired ./convert.py

This now works with the combined ConjunctiveGraph way of doing things.
It's still not great code, but it's only supposed to be temporary.
Changed the name of the input provenance graph.

It's now called prov_graph instead of myGraph. Hopefully this won't be confusing, as
the PROV ontology will always be called http://www.w3.org/ns/prov.owl