Fix 2q tomography

[stavros11]

Fixes #1032. I have not tested how this affect results from hardware, but it is quite clear that the fix is needed (otherwise the real and imaginary part are not compatible).

I also updated the docstring of the _fit method with more clear descriptions of the process steps, instead of the wikipedia link, which I think was more confusing than helping.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.30%. Comparing base (74d2625) to head (720a73e).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1035      +/-   ##
==========================================
- Coverage   97.33%   97.30%   -0.04%     
==========================================
  Files         120      120              
  Lines        9747     9747              
==========================================
- Hits         9487     9484       -3     
- Misses        260      263       +3     

Flag	Coverage Δ
unittests	97.30% <ø> (-0.04%)	⬇️

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Files with missing lines	Coverage Δ
...rc/qibocal/protocols/two_qubit_state_tomography.py	99.29% <ø> (ø)

... and 2 files with indirect coverage changes

[andrea-pasquale]

Thanks @stavros11 , LGTM.