Fix crash when switching groups on loaded multi-group segmentations

Summary:
Basically the fix will make the interpolator aware of situations where
there isn't any bookkeeping data structure yet for a group and
initialize one in that case before proceeding.

Fixes T29571

Test Plan:

• Load a multi-group segmentation
• Switch groups by clicking on a label that is not in Group 0. Shouldn't crash anymore.
• Activate 3-d interpolation and interpolate something in different labels in different groups (and time steps). Should still work and display the interpolation preview in all cases.

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T29571

Differential Revision: https://phabricator.mitk.org/D824