Add warning generators

[randomir]

Add generators of EnergyScaleWarning, TooManySamplesWarning and TooFewSamplesWarning warnings.

[conta877]

I am a bit worried about providing TooManySamplesWarning. without knowing the ground states of a problem creating a warning about drawing too many samples is not a great idea.

What if the first excited state is highly degenerate and you need a billion samples to see the ground state? - regardless of how we implement this specific Warning, it will throw an error for this specific example which is extremely misleading.

[arcondello]

There is a case to be made that that's a very poor QPU problem, though I agree distinguishing that case will be very difficult.

[conta877]

I disagree with that comment. It may simply be a hard problem with a hard landscape that other heuristics will fail too (possibly worse than the QPU).

[arcondello]

Fair enough, and it is definitely true that there will be many heuristics worse than the QPU at that problem. In any case, there is no way (that I know of) to detect that case, so maybe better to leave "too many samples" as a research topic.

[arcondello]

#255 adds EnergyScaleWarning and ChainStrengthWarning

[conta877]

do you want me to wait for this to be merged before adding TooFewSamplesWarning

[arcondello]

#255 has now been merged

[conta877]

so another question, the samples related warnings are applicable to classical solvers as well... you think they belong to dwave.system or dimod

[arcondello]

Let's keep them in dwave-system for now