T26974 Add function to extract 3D image for seed point selection

Summary:
Based on changes made in T27490, added a function to extract
the current selected 3d image of the current selected time point for
correct seed point selection.

Test Plan: Manual

Reviewers: kalali, O1 MITK Reviewer Group I, floca

Reviewed By: kalali, O1 MITK Reviewer Group I, floca

Subscribers: floca, thomass

Maniphest Tasks: T26974

Differential Revision: https://phabricator.mitk.org/D337