Fix correct_ans for Compute() when the result is a constant, and move creation of correct_ans_latex_string to Compute() so it properly corresponds to the correct answer string.  For answers not created through Compute, move creation of correct_ans_latex string to cmp() rather than cmp_parse(), since the latter may not be called (e.g., for blank answers), leaving no latex string.
Merge pull request #24 from dpvc/release/2.6

Fix correct_ans for Compute() when the result is a constant