Grader Feedback on Optimal Values

[ccoffrin]

Include optimal and best known solution values into the grader, so that we can check if the optimality flag is set incorrectly.

See the discussion here, https://www.coursera.org/learn/discrete-optimization/discussions/all/threads/PXqI_D_EEeeTwg79SJkWhg

[ccoffrin]

Another option would be to remove the optimality flag entirely.