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 (7167074) to head (60c75da).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #479 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 22 22 Lines 2093 2096 +3 Branches 346 347 +1 ========================================= + Hits 2093 2096 +3 ```

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