Add high-level merging class for RADOLAN method

[cchwala]

This PR adds a class based on MergeBase to run the RADOLAN adjustment provided by the rh_to_rw() function. As a first pragmatic step, I keep rh_to_rw() as it is and transform the data to pd.DataFrame as needed using the already existing functions and/or example code for that, plus some required adaptions.

TODO:

• add functioning MergeRADOLAN class
• adapt example notebook
• clean up example notebook and consider renaming it
• add tests

[cchwala]

This updated notebook from 030c5cc shows the same metrics as before in this notebook when the old lower-level function was used. The hexbin plot also looks very very similar. This means, it works! But tests need still to be added.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.30%. Comparing base (62f1b87) to head (5bdd29b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
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+ Coverage   95.37%   96.30%   +0.93%     
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  Files          11       11              
  Lines         800      840      +40     
==========================================
+ Hits          763      809      +46     
+ Misses         37       31       -6     

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[cchwala]

@eoydvin This is now ready and it would be good if you could have a brief look. Some comments:

1. The main change is the MergeRADOLAN class in merge.py.
2. There are test for this new class, which are okay IMO.
3. I had to also do some minor changes in some of the RADOLAN module files.
4. I added the RADOLAN method to the OpenMRG and OpenRainER high-level notebooks. RADOLAN seem show its skill with OpenRainER more (high PCC) than with OpenMRG. This is not (yet 😉) a competition, though. Results look okay for sure, that is most important currently.
5. There is also still the openMRG_case_RADOLAN.ipynb which was the only notebook where RADOLAN was used before. I also updated it so that it uses MergeRADOLAN. Currently the notebook is a bit obsolete, but in the future we could extend it to show some of the intermediate radar fields from the RADOLAN method and dig a bit in into the details. Hence, I would keep it in this PR.