Offer Scientio ontology/session as properties

[josephbirkner]

Currently, the Scientio session and ontology are exposed as global variables.

This is another block towards multi-context processes, and has to be accounted for in testing, by manually clearing the initialized event.

Instead, prop_sess and prop_onto, as well as sig_init will be offered by ravestate_ontology.

Note: Replacing the initialised event with sig_init will not be super easy, since initialised.wait has no right flank. Perhaps we could resolve this by implementing EmitStep(). See #18