Allow search tracing re-using existing IDP infrastructure

[Stef De Pooter]

At the moment there are two types of steps in a trace: - backtrack (jump to a lower decision level) - assign (truth assignment to an atom)

An assignment can be a choice or a propagation, but the trace does not tell us which one it is.

[Broes De Cat]

There is in fact enough information to also know the decisions: these are always the first propagation of any decision level. On commit https://bitbucket.org/krr/idp/commits/a1aad9293980b7a4518c03e724cec4ddfa64ce62 you can find a proposal of how to integrate the tracing in a cleaner way with idp (reading immediately into a structure).