Improve integration of nonlinear arithmetic into the arithmetic theory. (#6956)

Improve integration of nonlinear arithmetic into the arithmetic theory. (#6956)

The nonlinear extension used to be called only during model construction, a rather atypical place which lead to a series of subtle issues. This PR moves it into the postCheck method. To do that, a few other changes are necessary, most notably collectAssertedTerms and collectTerms are moved back from the model manager into the theory class.