Expose Solver in SolverJob

A SolverManager is very useful to avoid having to deal with threading. Unfortunately, using it also imposes a lot of restrictions on the interactions that can currently be had with the Solver. Currently, the only variable attributes that can be accessed from a SolverJob are the solver status and solving duration.

I also need to access the score calculation speed, and ideally get frequent updates. That could be solved by adding a pass-through method to SolverJob, but I believe the more flexible solution to be exposing the Solver altogether.

Since this is a smaller change, I would be happy to create a PR for it as long as there are no objections.