BQM-generating functions should specify the shape of the BQM

[arcondello]

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

[arcondello]

Also, for some of these algorithms the gap can be determined analytically, this information should also be exposed.