change histogram datatype

[ManuelHu]

very much WIP, just a quick sketch of some necessary changes to make the tests pass again

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.42%. Comparing base (f31bd0e) to head (ecb4a57).

Files with missing lines	Patch %	Lines
src/lgdo/lh5/_serializers/write/composite.py	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   80.38%   80.42%   +0.03%     
==========================================
  Files          47       47              
  Lines        3620     3647      +27     
==========================================
+ Hits         2910     2933      +23     
- Misses        710      714       +4     

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