FIX: enable radolan backend to work without GDAL

[kmuehlbauer]

No description provided.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (059e687) 86.44% compared to head (b56c890) 86.30%.

❗ Current head b56c890 differs from pull request most recent head 18784e8. Consider uploading reports for the commit 18784e8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #649      +/-   ##
==========================================
- Coverage   86.44%   86.30%   -0.15%     
==========================================
  Files          56       56              
  Lines        9510     9515       +5     
==========================================
- Hits         8221     8212       -9     
- Misses       1289     1303      +14     

Flag	Coverage Δ
notebooktests	0.00% <0.00%> (ø)
unittests	86.30% <96.15%> (-0.15%)	⬇️

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Files	Coverage Δ
wradlib/georef/polar.py	81.15% <100.00%> (+0.66%)	⬆️
wradlib/georef/rect.py	91.53% <100.00%> (+0.13%)	⬆️
wradlib/tests/test_georef.py	98.37% <100.00%> (-0.82%)	⬇️
wradlib/tests/test_vis.py	96.26% <100.00%> (-1.88%)	⬇️
wradlib/io/radolan.py	86.50% <87.50%> (+0.13%)	⬆️

... and 6 files with indirect coverage changes

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