Expose FitResult

[Astro-Kirsty]

Description
This PR is to resolve #5154

[bkhelifi]

Because this PR expose this class, I am wondering whether one should not describe the content of the FitResult class in the main docstring..

[registerrier]

Thanks @Astro-Kirsty .

Yes the docstring should give the correct info. This probably means that OptimizeResult and CovarianceResult need to be exposed too...

Regarding writing the FitResult to disc, we should check that we can fit everything in a yaml file. I suppose this is the easiest solution for this. This might be a second PR.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 75.23%. Comparing base (67efb7e) to head (f3eb789).
Report is 11 commits behind head on main.

Files	Patch %	Lines
gammapy/modeling/fit.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5255      +/-   ##
==========================================
- Coverage   75.27%   75.23%   -0.05%     
==========================================
  Files         234      234              
  Lines       35148    35181      +33     
==========================================
+ Hits        26458    26467       +9     
- Misses       8690     8714      +24     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[QRemy]

Thanks @Astro-Kirsty, I have only a minor comment about the change in the Registry class :

[QRemy]

Thanks @Astro-Kirsty, no further comment.

[registerrier]

Thanks @Astro-Kirsty . Looks good!