Named Graphs in dumps

[ch-sander]

https://github.com/epoz/shmarql/blob/5bda6004808d5dfe8768d8ce47a3b464838c9223/src/app/px_util.py#L60-L64

My px Store loads into a named graph. Queries are fine thanks to the use_default_graph_as_union flag.

Yet, dumps need to be TriG or my named graph needs to be hard coded in the from_graph argument.

Otherwise:

ValueError: A RDF format supporting datasets was expected, Turtle found

Not sure how to fix this with @epoz 's "big picture plans" for the app.