Cleaned up assertions and `print()`.
#66: Cleanup PEP8 issues: naming.
Cleaned up code structure.
#68: Cleanup and test errors.py.

Consolidated `DataFormatValueError` with `InterfaceError`.
Fixed that Excel format did not take sheet property into account.
#76: Add support for ODS files.
* #66: Cleanup PEP8 issues.
* #74: Cleanup project structure and build process.

Changed validation of coding guidelines from pep8 to flake8 and cleaned up the encountered issues.