Updating MC++ Interface Tests

[michaelbynum]

Summary/Motivation:

This PR updates a couple MC++ interface tests. In test_lmtd, instead of ensuring the convex underestimator does not change, I test that the convex underestimator is an underestimator at several points. I also check that the convex underestimator matches the original function at the lower and upper bounds of the variables. I do the same for the overestimators.

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[jsiirola]

Note that the new test_lmtd fails on the older release of MC++. Is that resolvable?

[michaelbynum]

That is unfortunate. Yes - it should be resolvable. Let me take a look.