[WIP] Import export fixes

This is not completely finished. I have to review some more paragraphs. But you can please have a look at the parts that have been reviewed yet.

ping @arbitbet What do you think?
Is this really the order in which the steps are executed? By looking at the code, I would have assumed that the order is totally different:

* the entity reader
* the processor
* the serializer
* the data converter
* the writer