Improved const correctness of PixelAccess methods

Summary:

• Add const overload for mitk::FastSinglePixelAccess()
• Add const overload for AccessPixel() (in QmitkAdaptiveRegionGrowingToolGUI.cpp)

Signed-off-by: Ralf Floca <r.floca@dkfz-heidelberg.de>

Test Plan: unit test

Reviewers: O1 MITK Reviewer Group I, kalali, kislinsk

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

Subscribers: kalali

Maniphest Tasks: T27368

Differential Revision: https://phabricator.mitk.org/D287