Hotfix/2.8 mathobjects

This pull request fixes several MathObject bugs, including:

*  Removing `correct_ans_latex_string` from the properties that are inherited when MathObjects are combined
*  Resetting the context flags set by `Compute()` to help it produce the `correct_ans_latex_string`
* Fix the `ijk()` method so that it respects the texStrings/normalStrings setting.