RGDA1 for RDFlib

RGDA1 (RDF Graph Digest Algorithm 1) combines the traces graph labeling algorithm with the Sayers and Karp RDF graph digest algorithm. This algorithm will produce canonical cryptographically unique identifiers for any RDF graph.
Renaming this is ok from my POV, but as it's exposed and others might depend on this, i'd keep the old `internal_hash(self)` around and just make it `return self.graph_digest()`. If you want you could add a `DeprecationWarning`