Add tmin and tmax for knmi stations

[martinvonk]

This pull request add the tmin, tmax data for the neerslag and meteo stations from the KNMI website.

[OnnoEbbens]

The dataframe with stations have mixed types for the tmin and tmax columns (bool, None and str). I want to check if the values are not False. Ruff raises an error, E712 Avoid equality comparisons to 'False'; use 'if not stations_df["tmin"]:' for false checks. The suggestion does not work because of the mixed datatypes in the column. Therefore I decided to ignore the ruff E712 error for this file.