Virtual full yield composite

Add a Ocean-side [VFY](https://docs.dwavesys.com/docs/latest/c_post-processing_5.html?highlight=virtual%20full%20yield#virtual-full-yield-chimera-solver) composite.

Something like
```
sampler = VirtualFullYieldComposite(DWaveSampler(), postprocessor)
```
where `postprocessor` is another [sampler](https://docs.ocean.dwavesys.com/projects/dimod/en/latest/reference/sampler_composites/api.html#dimod.Sampler) that accepts an initial state (see also https://github.com/dwavesystems/dimod/issues/491).

**Alternatives**
This could also be done in [dwave-hybrid](https://github.com/dwavesystems/dwave-hybrid).