Fix fit Rabi

[Edoardo-Pedicillo]

This PR fix Rabi fit and refactor Rabi utils.
Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.
• Compatibility with Qibo modules (Please edit this section if the current pull request is not compatible with the following branches).
  • Qibo: master
  • Qibolab: main
  • Qibolab_platforms_qrc: main

[codecov]

Codecov Report

Merging #651 (4e2863f) into main (8398e2b) will increase coverage by 0.02%.
Report is 27 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #651      +/-   ##
==========================================
+ Coverage   96.03%   96.06%   +0.02%     
==========================================
  Files         106      106              
  Lines        7347     7339       -8     
==========================================
- Hits         7056     7050       -6     
+ Misses        291      289       -2     

Flag	Coverage Δ
unittests	96.06% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/qibocal/auto/task.py	100.00% <ø> (ø)
...bocal/protocols/characterization/rabi/amplitude.py	94.94% <ø> (ø)
...rotocols/characterization/rabi/amplitude_signal.py	93.75% <ø> (ø)
.../qibocal/protocols/characterization/rabi/length.py	95.91% <100.00%> (ø)
...l/protocols/characterization/rabi/length_signal.py	97.50% <100.00%> (+2.50%)	⬆️
...c/qibocal/protocols/characterization/rabi/utils.py	100.00% <100.00%> (ø)

[andrea-pasquale]

Thanks @Edoardo-Pedicillo, LGTM.

[andrea-pasquale]

If you can could add a quick test for this function to increase coverage.