Propagate the `info` field from the child sampler

[JoelPasvolsky]

Adding this specifically for the FixVariablesComposite (in addition to the generally known issue of propagating info) because for this one, once variables are dropped, the embedding context becomes very helpful, not just the timing info.

[jackraymond]

This should be required as a default in all composites inheritng whereever possible the child info fields; greedy post-processing, clique+virtual-graph, and spin-reversal transform composites come to mind as cases where information is often unnecessarily destroyed (things like qpu timing information).
For my recent work with D-Wave hybrid I had to hack together this work around: https://github.com/dwavesystems/dwave-hybrid/blob/master/hybrid/samplers.py#L114 .
Not sure where, but there should be some other open-issues around