Use Python modules and refactor utils files

[SvenLieber]

Currently we have Python scripts in different folders, e.g. KBR-related pre-processing scripts in the data-sources/kbr folder or post processing scripts in the data-integration folder.

Helper functions exist in several generic utils.py files in those folders. However, a few functions are defined in different utils.py files.
We should refactor the Python code to consist of modules. Helper functions from generic utils files should be refactored to reside in more thematic modules.

[SvenLieber]

A first bit of refactoring was done in the following commit, splitting up the data-integration utils.py file into several more thematic utils_*.py files 684926d

[SvenLieber]

There are still many utils files with ISBN processing #292