Failing tests in uncertainty tool box with new NL writer

[andrewlee94]

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With the transition to the new Pyomo NL writer (done in PR #997), we started seeing failures in the uncertainty toolbox (idaes/apps/uncertainty).

The one known issue that has not been fixed is test_quantify_propagate_uncertainty_NRTL_exception which has been xfailed for now. The reason appears to be that the test problem is not actually infeasible, and that it just failed to solve in the old NL writer. Looking at the tests, a lot of them look like they should be broken down into smaller elements that can be unit tests, as at the moment a lot of hte testing is done with high-level integration tests (which also take a long time to run).