Improve logging in datahandling

At the moment, we are using mostly logging.info and its siblings for the logging. This works reasonably well, but could be improved by getting a specific logger instance per module as is common in Python projects.

As a first step, because it was already used in some debugging efforts, we can do this for the datahandling module. This should be extended to logging in all modules and combined with the efforts in !146 (closed) to make the overall logging experience more enjoyable.