Clarify/update SplatComposer behavior

[randomir]

Note:

• hybrid.SplatComposer does not sort output samples by energy

  • subsequent hybrid.ArgMin with the default key="samples.first.energy" might not pick the best sample by energy
  • easily fixed by sorting after compose, with e.g: hybrid.SliceSamples(), or a more explicit hybrid.SliceSamples(sorted_by='energy').

• hybrid.SplatComposer does not sort input subsamples by energy before composing with a smaller samples set

  • if subsamples generator does not produce sorted sampleset, and is seeded by, say, a single initial state, the output sampleset (containing just one sample) might not be the best

  • easily "fixed" by sorting subsamples generator output

  • but a better fix might be to explode the input sample with hybrid.AggregatedSamples(aggregate=False) before composing with Splat, followed by a truncate with hybrid.SliceSamples(1). So, something like:

    subsampler = (
        hybrid.AggregatedSamples(aggregate=False)
        | hybrid.SimulatedAnnealingProblemSampler()
        | hybrid.SplatComposer()
        | hybrid.SliceSamples(1, ordered_by='energy')
    )
    

    would make sure the best sample is generated, accounting for boundary conditions.

(Originally raised offline by @jackraymond)