FIX: add capability to decode old DX header, add test

[kmuehlbauer]

closes #455

[codecov]

Codecov Report

Merging #467 into master will increase coverage by 0.00%.
The diff coverage is 96.66%.

@@           Coverage Diff           @@
##           master     #467   +/-   ##
=======================================
  Coverage   90.54%   90.55%           
=======================================
  Files          35       35           
  Lines        7386     7400   +14     
=======================================
+ Hits         6688     6701   +13     
- Misses        698      699    +1     

Flag	Coverage Δ
wradlibtests	89.31% <96.66%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
wradlib/io/radolan.py	95.73% <96.66%> (-0.13%)	⬇️

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