Make everything installable via PyPI

[cmungall]

• [ ] rewrite relation-graph in rust and use PyO3 bindings
• [ ] use PyO3 to wrap rdftab

As an alternative to wrapping rdftab is to directly load the statements table in Python. This will be slower, but it should be very straightforward if we skip loading of the stanza field, which we don't use. It will also have the advantage that we don't need to do transformations to RDF/XML using riot or robot.