Corrected SceneIO test to regard preconditions of mitk::PropertyList::SetProperty

Summary:
+ added tests to PropertyListTest to check the preconditions
+ added information in the class documentation

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

Test Plan: unit test

Reviewers: MITK Reviewer Group I, kislinsk

Reviewed By: MITK Reviewer Group I, kislinsk

Subscribers: kislinsk

Maniphest Tasks: T25441

Differential Revision: https://phabricator.mitk.org/D168