Model.to_petab: store parameter estimates as nominal values

[dilpath]

Should make it easier to then simulate the full parameter vector, e.g. via petab_select.Model.to_petab()[PETAB_PROBLEM].x_nominal.

[codecov-commenter]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (f3a28de) 57.44% compared to head (87379a5) 57.50%.
Report is 2 commits behind head on develop.

Files	Patch %	Lines
petab_select/ui.py	12.50%	7 Missing ⚠️
petab_select/model.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #77      +/-   ##
===========================================
+ Coverage    57.44%   57.50%   +0.06%     
===========================================
  Files           14       14              
  Lines         1478     1485       +7     
===========================================
+ Hits           849      854       +5     
- Misses         629      631       +2     

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