Require peakfinder_σ_scaled in sipm_simple_calibration to be an integer

[fhagemann]

In 8c62c6a, the explicit Int declaration of peakfinder_σ_scaled was removed.
Therefore, we should check if peakfinder_σ_scaled can be converted to an integer (despite fixing the typo in rount)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 28.37%. Comparing base (3d9f9bd) to head (12d0e05).

Files with missing lines	Patch %	Lines
src/sipm_simple_calibration.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   28.39%   28.37%   -0.02%     
==========================================
  Files          37       37              
  Lines        3564     3566       +2     
==========================================
  Hits         1012     1012              
- Misses       2552     2554       +2     

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[theHenks]

This looks fine to me, can this be merged or do you want to do further tests @fhagemann?