Things need to be a bit cleaned up / tidied up / refactored

• There's an opportunity to implement a testable function to find the beginning of the week X. The function is available in Python 3.7 and is backported in the Netherlands calendar, but it's spread out in several classes, so I think it would be worth centralize this mechanic in the core module.
• Test coverage has decreased and it can be improved (Improve coverage where it's possible #546)
• I've seen a few lines to remove in the test classes.