Remove (or de-emphasize) dimod.reference samplers

[randomir]

It's misleading/confusing to provide simulated annealing sampler (and exact solver) as part of dimod library. The problem is not just in code organization, but the fact the dimod.reference.SimulatedAnnealingSampler is a paedagogical, Python (slow) implementation of SA. Users are much better off using the optimized C++ version available in dwave-neal package.

Since dimod.reference.SimulatedAnnealingSampler is promoted in documentation and training materials, users (even library creators) are starting to use it for wrong use cases (e.g. see pyqubo.utils.solve_qubo()).

Since dimod.reference.* samplers demonstrate how to write a custom sampler, target audience, supposedly, are developers. Therefore, better location for them is out of dimod namespace, probably in level with tests. They can still be documented (included in main docs), just as long they are un-importable.

The location (namespace) for samplers is TBD, but to prevent compatibility breaking, we might (transitionally) import neal.SimulatedAnnealingSampler in dimod.