Add Vinecop::fit()

[tvatter]

• Adds the possibility to fit each pair-copula parameter without selecting the corresponding families.
• This is kinda useful and would also How to use sets of families for the BicopFamily class pyvinecopulib#63

[codecov]

Codecov Report

Attention: Patch coverage is 98.95833% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.79%. Comparing base (e132e19) to head (3e751e0).
Report is 2 commits behind head on dev.

Files with missing lines	Patch %	Lines
...clude/vinecopulib/vinecop/implementation/class.ipp	98.82%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #584      +/-   ##
==========================================
+ Coverage   95.77%   95.79%   +0.02%     
==========================================
  Files          44       44              
  Lines        5893     5973      +80     
==========================================
+ Hits         5644     5722      +78     
- Misses        249      251       +2     

Files with missing lines	Coverage Δ
...inecopulib/vinecop/implementation/fit_controls.ipp	97.58% <100.00%> (+0.43%)	⬆️
...clude/vinecopulib/vinecop/implementation/class.ipp	96.18% <98.82%> (+0.39%)	⬆️

... and 1 file with indirect coverage changes

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