Pass down a provenance graph at ingest time in `sema.commons.store`

[cedricdcc]

Allow passing a provenance graph during data ingestion.

[cedricdcc]

Links to #21

[marc-portier]

I suppose the use case is the following:

• I have some triples
• and I have some triples about how those triples came to be

The idea would be to have both passed into one call to add() these.

Questions though:

• would there be a reason not to just merge them and store them together? --> if so could the client just do that prior to passing them? or can we simply have a list of graphs as argument for convenience so we iterate over the ingestion of them all
• would these need to be in a separate - but associated named-graph?