FIx: make dtypes consistent

[codecov-commenter]

Codecov Report

Merging #89 (2fd64b2) into master (de89517) will not change coverage. The diff coverage is 100.00%.

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@@            Coverage Diff            @@
##            master       #89   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          636       635    -1     
=========================================
- Hits           636       635    -1     

Flag	Coverage Δ
pytest	100.00% <100.00%> (ø)

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Files	Coverage Δ
src/pygenstability/constructors.py	100.00% <100.00%> (ø)

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