DEICH-6215 - DFB using MARC21
In its current form the code is usable, however the mapping part is kind of a monolith. I suggest code refactoring to the approach we talked about: small mapper utility classes getting the whole record (not just a field), and fetch the dat needed to create subjects, contributors etc...
Tests should be also made uniform when it comes to creating the test models. We should use the text block + parse approach all places.