Updating CDAO library to remove dependency on librdf. Using RDFlib instead.
Write triples directly to file instead of going through rdflib. This avoids storing entire graph in memory.
CDAO trees and clades have a [(predicate,object)] `attributes` list containing any additional triples with that tree/clade as subject.
Use @base for more concise URLs.
Use rdflib.URIRefs for URIs in yielded statements; if part of a statement is a string, treat it as raw RDF.
Only add an edge annotation/branch length if the clade specifies a branch length.