Transforms on SampleSets with unresolved futures

**Application**

Example:
In #429 if we wish to relabel a sampleset that has not yet been resolved, we need to block on the future before applying the relabelling. This makes sense in the copy case but not in the in-place case.

Example:
https://github.com/dwavesystems/dimod/blob/d60eeb86d3baf9387a4540d0a6697ed1b193298d/dimod/core/sampler.py#L150
which causes it to block in some cases and not in others, depending which `.sample` method was used.

**Proposed Solution**
We can nest futures. This could either be done in the relevant methods (e.g. `SampleSet.relabel_variables`) or we could make a `SampleSet.apply_transform` method or similar.

**Alternatives Considered**
* We could make the SampleSet wait for the future to resolve *on construction* which would simplify things considerably, potentially at the cost of performance.

**Additional Context**
* https://github.com/dwavesystems/dwave-system/issues/91