consolidate_start_end_years does nothing

This concerns this function the backend:

https://github.com/CentreForDigitalHumanities/I-analyzer/blob/83e49633c507ff7772786247f6182b2ccd6975fc/backend/addcorpus/python_corpora/corpus.py#L519-L547

This function is used in the dutchnewspapers and times corpus definitions. The docstring is clear, but does not seem to match the body of the function, which appears to have no effect. (No assertions, no return value, no side effects.)

Implicitly, the function performs type-checks on the provided variables. We recently made some changes in the types for the time range of a corpus, e.g. https://github.com/CentreForDigitalHumanities/I-analyzer/pull/1742, which required some changes to this function. Perhaps something got lost here?

**To do:**
- Add a unit test for this function based on the docstring
- Correct the function if the test fails