Add/extend exponential decay fit models
Summary: This differential is in tandem to the ongoing review of D512. The ExpDecayOffsetModel introduced in D512 is moved to the general ModelFit module, instead of being in a plugin-specific module. T2DecayModel is fixed so the function calculation actually works, and is slightly extended.
Test Plan: Not sure if there is a direct way to test this. At the moment, the only way is to test the first steps of the workflow related to D512
Reviewers: O3 MITK Reviewer Group II, floca
Reviewed By: O3 MITK Reviewer Group II, floca
Subscribers: floca
Differential Revision: https://phabricator.mitk.org/D596