Allow flux_bias_offset, anneal_offset, et al to be specified in terms of source variables

[arcondello]

Similar to: https://github.com/dwavesystems/dwave-system/blob/f7926eccf117c3a9959a0b8dc86f2c46902ff344/dwave/system/composites/embedding.py#L252

See also #217

[zsolttabi]

Since the offsets play a huge role when dealing with long chains, I think it would make sense to reflect this logic in the API (maybe a utility accepting an embedding and some mapping function?).

Also, is it possible, that some embeddings might not allow for setting all physical qubits in the same chain to the same offset? If so, there should be a way for the embedding algorithm to take this information into account.