improved const correctnes and documentation; added callback feature for load functions to the public interface.

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

removed comment "polution".

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

fixed T22780; implemented white list reader option functor.

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

Test Plan: new unit test; code review

Reviewers: O1 MITK Core!

Maniphest Tasks: T22780

Differential Revision: https://phabricator.mitk.org/D46