Improve consistency of rational expression module

Using .regex() will now work with Term and RationalExpression objects with and without true.

Fix adding_and_subtracting_rational_expressions_3.html and a few other exercises so it works with the new system. I've updated the rational expression unit tests and checked everything passes. I've tried the changed exercises and some other rational expression exercises multiple times and they seem to work, so fingers crossed. Apologies for the problems. I should never have started using the Term object for single term expressions.