fix: respect fixed parameters in n_dof calculation

[lorenzennio]

Solves #478 by passing fixed parameters to model_utils.unconstrained_parameter_count.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (19ce6a1) to head (0a379af). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #479   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         2094      2096    +2     
  Branches       347       347           
=========================================
+ Hits          2094      2096    +2     

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[MoAly98]

Hi @alexander-held, I think this is ready and non-breaking change, unless you did tests which hint otherwise?