Handling Default Graphs, Quad Files and the asignment of URIs

[white-gecko]

Forked from #81 .

(@Kyrosh)

• we only considered .graph files with one graph for named graphs (basically we should also store triples instead of quads) but this wont be true for the unassigned.nq file which should hold all unassigned graphs. We could also insert each named graph in its own file (also nice because of the blob cache for instance creation), but how will an uri be transformed to a filename?
• In the last case we could also use generic names (less readable for manual git usage) and offer an interface to rename those files. In manual mode with graphfiles settings the user will be lost since he can't see all file <> graph assignments except we offer the same interface as a CLI.

[white-gecko]

I like your suggestion with one file per graph as a clean intermediate solution until we have sorted out the rest of the problem.

[white-gecko]

We have dropped the unassigned.nq and the default graph handling is already covered in #30