Implement logging

[paulmueller]

This is important for understanding why a file format is not working (see e.g. #29).

When a file format is not supported, it should log a reason (e.g. a traceback) into DEBUG so we can understand what went wrong.

[paulmueller]

Normal logging scheme should be used:

# at beginning of each file:
import logging
import traceback

logger = logging.getLogger(__name__)

[....]

def is_file_format_supported():
    try:
        ...
    except BaseException:
        logger.debug("File format not suppoted. Traceback follows.")
        logger.debug(traceback.format_exc())