BQM-generating functions should specify the shape of the BQM

In #105 we added some lines to the `travelling_salesman_qubo` that specify the shape (number of variables, number of interactions) of the generated qubo relative to the original graph. We should do this for all of the generating functions.

These should also come with tests.

**Example**
https://github.com/dwavesystems/dwave_networkx/blob/dae190a5530d9f06fca89bf7e3737c06c7e6ab5a/dwave_networkx/algorithms/tsp.py#L113