Fix Compute() to use stringified version of the original string for the correct answer, just like it does for the latex string.  This will make sure they are consistent (now that the latex string is the one actually shown).  This also fixes the problem of how to handle alternate formats like commas for decimal separator, and backward brackets for intervals.
Merge pull request #177 from dpvc/patch-compute

Fix Compute() to create correct answer string from the parsed answer