Allow distinct blank node contexts from one NTriples parser to the next (#980)

[mwatts15]

Partial solution to #980. Allows to re-use a blank nodes across calls to parse while still allowing to keep the same context when desired.

Didn't touch derived parsers (NT, NQuads). Can do if the proposed interface is acceptable.

[coveralls]

Coverage increased (+0.04%) to 75.802% when pulling 92f66b3 on mwatts15:feature/#980-ntriples-blank-node-collisions into 037ea51 on RDFLib:master.

[coveralls]

Coverage increased (+0.04%) to 75.802% when pulling 92f66b3 on mwatts15:feature/#980-ntriples-blank-node-collisions into 037ea51 on RDFLib:master.

[nicholascar]

Thanks @mwatts15, looks good! So, you're slowly chipping away at the long-standing RDFlib issue of BNodes? I do like the idea of passing a context to make BNs the same, if that's what a user wants. Or just to leave them defined per-graph by leaving out a context. Gives the users options.

[mwatts15]

So, you're slowly chipping away at the long-standing RDFlib issue of BNodes?

Eh. I don't really like blank nodes, but I think people will want to use them in what I'm building, so I'd like to make it less painful for myself. Also, have seen a few PRs on RDFLib that I could review, but it's easier for me to just give an example of what might be a better approach.

Going to make another PR since this one doesn't completely address #980 (doesn't handle the format='nt' option to parse). Will also cover NQuads. Implementation will be different to support that.