Check that stitch cascades through each penaltymodel

[m3ller]

Application The stitch function generates a BQM's penaltymodel by cascading thorough Cache, LP, MIP, and finally, MaxGap.

• Make a test to verify that this cascade is happening in the expected order.
• Make sure that the simpler BQMs are being caught earlier in the cascade (i.e. a problem that can be solved with LP should not be able to cascade down to MaxGap).

Proposed Solution Make problems of varying "difficulty" so that we can observe that the proper penaltymodels are catching said problems. (Ex. LP should be able to catch an OR-gate, but not an XOR-gate. XOR-gate needs auxiliary variables and LP does not handle that, therefore the problem should be passed to MIP.)