Adding operator sinkhorn operator.

[vprusso]

Closes #848

[codecov]

Codecov Report

Attention: Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.8%. Comparing base (14549b5) to head (96967eb).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
toqito/matrices/operator_sinkhorn.py	95.9%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #849     +/-   ##
========================================
- Coverage    97.8%   97.8%   -0.1%     
========================================
  Files         168     169      +1     
  Lines        3259    3308     +49     
  Branches      800     723     -77     
========================================
+ Hits         3189    3236     +47     
- Misses         46      47      +1     
- Partials       24      25      +1     

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