Reactivate / fix exponentiation in formula parser

Summary: In the formula parser, exponentiation was defined as a primary followed by ^ and a factor, inside the definition of factor itself. Instead, it should connect two primarys, or exponentations can get lost while parsing. Fixes T20705

Test Plan: Do a generic param model fit and use ^ in the formula. The fit should work correctly.

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Subscribers: kislinsk

Maniphest Tasks: T20705

Differential Revision: https://phabricator.mitk.org/D878