test: add test file from DWD

[Nerixyz]

What I did

This adds a test file for data from DWD. This is extracted from a file in https://opendata.dwd.de/weather/weather_reports/synoptic/germany/. The files there contain multiple BUFR messages wrapped in GTS messages (plus an 8 byte length + 2 null bytes at the start). Since there are a lot of entries for the B and D tables from DWD (848/439), I only included the ones needed for this test.

As the test code would be similar to the JMA code, I factored out the part that actually reads the data into a module.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.04%. Comparing base (1d908cf) to head (37af525).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #10       +/-   ##
===========================================
- Coverage   84.61%   43.04%   -41.57%     
===========================================
  Files           5        6        +1     
  Lines         416     1136      +720     
===========================================
+ Hits          352      489      +137     
- Misses         64      647      +583     

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[ciscorn]

perfect. thanks!