Add the union-default-graph option to make SPARQL queries use the union of all graphs as the default graph

[ch-sander]

see https://github.com/oxigraph/oxigraph/pull/975 and https://github.com/oxigraph/oxigraph/issues/973 This is currently a feature of the CLI, not sure about pyoxigraph supporting it in the same way.

[ch-sander]

would be an easy fix, thanks to use_default_graph_as_union:

query = """
SELECT ?s ?p ?o ?a
WHERE {
    ?s ?p ?o .
}
LIMIT 10
"""
results = store.query(query=query,use_default_graph_as_union=True)