Some KNMI fixes

[dbrakenhoff]

• Fixes issue where current station has no data between start/end, and then is no longer present in stations dataframe, causing a KeyError.
• Fixes issue so stations dataframe is not filtered when start and end are None.
• Fixes issue for getting stations with meteo_var=slice(None), list all stations that have at least one of all possible meteo_vars. (Before this change hpd.io.knmi.get_station_name(739) was not working).

[OnnoEbbens]

Hi Davíd, I shuffled the code a bit because I think that works better. Can you have a look? I already approved so if you like it you can merge the PR.

[dbrakenhoff]

Nice, this is even better. Merging.