Add SQLite/SemSQL as an export format

[gouttegd]

The ODK should allow to produce release artefacts that are SQLite files conforming to the INCATools/semantic-sql schema, alongside the other “traditional” formats (OWL, OBO, etc.).

semsql itself and the tools it depends on (rdftab, relation-graph) are already part of the ODK, so it should be relatively straightforward (I hope I won’t regret writing that!).

(rdftab and relation-graph are only present in ODKFull, though – that’s annoying if we want the SemSQL artefacts to be produced by default, because I am keen to keep the standard workflows depending only on ODKLite.)

[cmungall]

I regret to inform you that there hasn't been much progress on:

• Make everything installable via PyPI semantic-sql#41

Which would make this easier

[gouttegd]

Well, as I said, the tools we need are already there anyway (we build rdftab from source and we have a running version of relation-graph). Making those tools easier to install would still be a good thing overall (for other users), but for the ODK it’s too late. :)

Now we just need to use them.