Better verifier

[etwest]

• GraphSketchDriver builds a verifier and supplies it to the algorithm.
• GraphSketchDriver can check its verifier against a "ground truth" verifier. For example, from a cumulative file.
• GraphSketchDriver now passes an integer query code to the algorithm's has_cached_query(int) function so that the algorithm can determine on a query type basis whether it has an answer cached. For example, we can currently cache a single spanning forest but not k spanning forests.
• File and Matrix verifiers combined into a single verifier class.
• Sketch::calc_cc_samples always returns a number of samples >= 18 to avoid errors for small graphs in our unit tests
• Added spanning forest verification functions to the verifier.
• Added unit testing for the verifier, for spanning forest generation, and for insert only streams.