Fix preprocess allowed_symbols

[derNarr]

Fixes #245 .

Changes proposed in this pull request:

• removes the wrong documentation
• adds a special value for all symbols ("all"), which is also the default
• removes the old default, which leads to no allowed symbols

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (be6c481) 83.75% compared to head (e4ac99f) 83.65%.

Files	Patch %	Lines
pyndl/preprocess.py	33.33%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #247      +/-   ##
==========================================
- Coverage   83.75%   83.65%   -0.11%     
==========================================
  Files           9        9              
  Lines        1410     1413       +3     
==========================================
+ Hits         1181     1182       +1     
- Misses        229      231       +2     

Flag	Coverage Δ
unittests	83.65% <33.33%> (-0.11%)	⬇️

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