refactor: list conversions

[alexander-held]

Minor refactoring for list conversions in fit and model_utils.

* refactored list conversions in fit and model_utils modules

[codecov]

Codecov Report

Merging #268 (0df0b7c) into master (5220056) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #268   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        19           
  Lines         1798      1798           
  Branches       278       279    +1     
=========================================
  Hits          1798      1798           

Impacted Files	Coverage Δ
src/cabinetry/fit/__init__.py	100.00% <100.00%> (ø)
src/cabinetry/model_utils.py	100.00% <100.00%> (ø)

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