For retention parameter estimator

[JanLeppert]

• added some functions for usage in the RetentionDataEstimator.jl package
• changed sqrt in local pressure functions to avoid DomainError by using real(sqrt(x))
• added the parameter k_th (a maxima threshold for the retention factor) to the Options structure
• corrected the reading of the CAS number from ChemicalIdentifiers.jl to have always two digits in the second part (add a leading 0, if ChemicalIdentifiers.jl has only one digit for the second part)

[codecov-commenter]

Codecov Report

Merging #70 (bf8f927) into main (a1801f7) will decrease coverage by 0.66%. The diff coverage is 60.37%.

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
- Coverage   82.98%   82.31%   -0.67%     
==========================================
  Files           5        5              
  Lines        1087     1097      +10     
==========================================
+ Hits          902      903       +1     
- Misses        185      194       +9     

Impacted Files	Coverage Δ
src/Model.jl	65.00% <57.14%> (ø)
src/GasChromatographySimulator.jl	93.59% <64.00%> (-1.89%)	:arrow_down:

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