BQM should store the lowest known energy

It might be useful for samplers to be able to "annotate" binary quadratic models with data such as the lowest energy found by any solver and/or the state(s) that found that energy. This lowest energy could potentially come with a certificate marking it as a ground state (when that's possible).

**Uses**
* The current best state(s) could be used to see other solvers that accept an initial state (see also #491)
* Useful for benchmarking, because it provides a target and different samplers could be rated on time-to-target

**Additional Context**
This data could just be stored in the `BQM.info` dict.