Add and test a helper function for causal necessity constraints

[eb8680]

The design in #240 and previous iterations includes a number of new components on which the overall correctness of causation and explanation query implementations depends. To reduce the difficulty of reasoning about these implementations and make incremental progress toward landing causation and explanation queries in master, we should pull out these components and specify and test them in isolation.

One of these components is a helper function for defining log-factors (used in conjunction with the Factors handler added in #238) that enforce the constraint that the value of a consequent in a counterfactual world should differ from its value in the factual world. In the draft design in #240 this function is called consequent_differs:

def consequent_differs(...) -> Callable[[Tensor], Tensor]: ...