Implement center of mass calculation when label is clicked

Summary: Previously, when a label in the segmentation plugin was clicked, the crosshair simply jumped to the position of the index in the middle of all indexes of that label. I now actually implemented the calculation of the center of mass. Fixes T29354

Test Plan: Double-click on a label name in the segmentation view. The crosshair should now jump to the center of that label. (does not work for dynamic 4D segmentations)

Reviewers: O1 MITK Reviewer Group I, kalali

Reviewed By: O1 MITK Reviewer Group I, kalali

Subscribers: kalali

Maniphest Tasks: T29354

Differential Revision: https://phabricator.mitk.org/D751