some refactoring + starting on the basic tracing tests based on
post-processing the saturation stage. The first test basically
recursivly unwind the inferences for all subsumers in all contexts and
checks that we end up at premise-free inferences (initialization).
fixes to the basic tracing test (avoiding loops, etc) + fixing the
bridge inference: it needs to store a reference to the context where the
conclusion was produced.
a better tracing of property composition inferences (those are complex
because involve 3 contexts: the backward link source, the forward link
target, and the context where the inference actually happens). Now
PropertyChains.owl passes the tracing tests. Next: a proper tracing
of contradictions.