Update reading knmi

[OnnoEbbens]

Removed the KNMIObs and created 3 new objects instead:

• MeteoObs for any type of meteorological observation
• PrecipitationObs a child class of MeteoObs for precipitation data
• EvaporationObs a child class of MeteoObs for evaporation data

Modified the knmi module in such a way that missing precipitation data from a meteo station can now be filled with nearby data of a precipitation (neerslag) station.

[dbrakenhoff]

Looking good! I like the new objects being more explicit about the contents of the data. I left some comments, mostly style things and docstrings and I also modified a few style things here and there.

I really like the overview in the notebook including the comparison between the different datasets from KNMI.

With the above fixes, seems good to go.

[dbrakenhoff]

I attempted to reduce the number of codacy issues with docformatter and autopep8, hopefully that helps somewhat.

[OnnoEbbens]

Thanks a lot for checking all this. I will try to answer your comments later today.

One last thing that I just found out and is super annoying. The fill_measurements function can now combine precipitation data from meteo- and neerslagstations. Only meteo stations record daily precipitation starting at 00:00 and neerslagstations record precipitation starting at 08:00 UTC. Combining them introduces errors. In theory we could use the hourly precipitation data from the meteo stations to get daily precipitation starting at 08:00 so we can combine them with neerslagstations. This is however a bit annoying to implement so I won't be able to fix this today :/