Consistent handling of initial states in Samplers/Composites

**Application**
There are multiple dimod samplers that handle seeding with initial states.

See:
* https://github.com/dwavesystems/dwave-neal/issues/56, https://github.com/dwavesystems/dwave-neal/pull/58, https://github.com/dwavesystems/dwave-neal/issues/59
* https://github.com/dwavesystems/dwave-tabu/pull/42, https://github.com/dwavesystems/dwave-tabu/pull/41
* #483
* https://github.com/dwavesystems/dwave-system/pull/96

Also noteable that the DWaveSampler in [dwave-system](https://github.com/dwavesystems/dwave-system) handles initial states ([docs](https://docs.dwavesys.com/docs/latest/c_solver_1.html#initial-state)).

I think it would make sense to define some standard behaviour across Ocean.