Refactored PoissonExactSolution to use the modern ExactSolution facilities; similarly with PoissonBilinearForm.  Deleted a bunch of legacy Poisson implementations.  Overall project is getting rid of Sacado dependency.  (Still need to do this for ConfusionManufacturedSolution.)

Separately, working on diagnosing some issues in minimum rule.  Currently, these tests crash, so they're disabled in the present push to the repo.