Reader returns list of read files. IOUtil checks this list to avoid duplicate reads

Test Plan: This has no effect if a reader does not actively fill the list of read files. So everything should work as before. TZhis is covered by the current tests.

Reviewers: O1 MITK Core, goch

Reviewed By: O1 MITK Core, goch

Subscribers: goch

Maniphest Tasks: T23452

Differential Revision: https://phabricator.mitk.org/D74