SCALAR: Improve handling skipping of empty data

[gutzbenj]

This PR changes how we work with empty data on skip_empty=True.

When using filter_by_rank now empty stations are skipped based on actual percentage of values which is calculated for all requested parameters per station.

Thresholds for it can be defined with skip_criteria which is one of "min", "mean", "max", where

• "min" means the lowest found percentage of all parameters
• "mean" means the mean value of actual percentages of all parameters
• "max" means the highest actual percentage of all parameters

Because we don't know from the start which station has enough values, after collecting all data there is a property on the values class named df_stations that holds all stations that were finally collected, so that you can get a sense for the data.

[codecov]

Codecov Report

Base: 89.65% // Head: 89.65% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (c2dac57) compared to base (568cc07).
Patch coverage: 92.30% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #889   +/-   ##
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  Coverage   89.65%   89.65%           
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  Files         104      104           
  Lines        7615     7666   +51     
  Branches      952      965   +13     
=======================================
+ Hits         6827     6873   +46     
- Misses        599      601    +2     
- Partials      189      192    +3     

Flag	Coverage Δ
unittests	89.65% <92.30%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
wetterdienst/core/scalar/tools.py	91.66% <ø> (ø)
wetterdienst/core/scalar/request.py	86.63% <79.16%> (+0.23%)	⬆️
wetterdienst/core/scalar/values.py	82.47% <93.33%> (+0.53%)	⬆️
wetterdienst/core/scalar/result.py	90.19% <100.00%> (+1.30%)	⬆️
wetterdienst/provider/dwd/mosmix/api.py	86.54% <100.00%> (-0.09%)	⬇️
wetterdienst/provider/dwd/observation/api.py	87.66% <100.00%> (-2.12%)	⬇️
wetterdienst/provider/dwd/observation/fileindex.py	100.00% <100.00%> (ø)
wetterdienst/settings.py	89.28% <100.00%> (+0.39%)	⬆️
wetterdienst/ui/cli.py	73.84% <100.00%> (+0.10%)	⬆️
wetterdienst/ui/core.py	62.60% <100.00%> (ø)
... and 4 more

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