fix wrong call by timestep instead of timepoint

Summary: Fixes T29330 (as well as a problem in D739 and perhaps other tasks). A wrong call to SelectImageByTimeStep instead of SelectImageByTimePoint caused the wrong segmentation image to be used in preview tools on dynamic 4D segmentations. I tested it for the two mentioned cases and it seems to fix the problem.

Test Plan: See T29330 / D739. Create a dynamic 4D segmentation on an appropriate image and use either the picking tool or grow-cut on various timesteps (especially not the first and last). The tools should now use the correct reference segmentations, from the current time.

Reviewers: O1 MITK Reviewer Group I, kalali

Reviewed By: O1 MITK Reviewer Group I, kalali

Maniphest Tasks: T29330

Differential Revision: https://phabricator.mitk.org/D747