refactor QuantumNetwork to support general quantum networks

[Canoming]

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

I've refactored the QuantumNetworks to support general quantum networks and make it more compatible with usual tensor contraction methods.

To make things clear, I made a class diagram for the future development of the module

[Canoming]

The build of the wheel fails after I merge the master into this branch. Not sure what causes the problem

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.82%. Comparing base (c132711) to head (d3798e1).
Report is 265 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1244    +/-   ##
========================================
  Coverage   99.81%   99.82%            
========================================
  Files          72       72            
  Lines       10520    10659   +139     
========================================
+ Hits        10501    10640   +139     
  Misses         19       19            

Flag	Coverage Δ
unittests	99.82% <100.00%> (+<0.01%)	⬆️

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

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

[AlejandroSopena]

Thanks @Canoming. All the methods seem to be implemented correctly.

Regarding the documentation, I have noticed that the new classes QuantumComb and QuantumChannel need to be added to qibo.srt so that they appear on the website. The file README.md is not included in any toctree. I think the best place to add it is in advancedexamples.rst.

I only have a few minor suggestions at the code level:

[renatomello]

@Canoming I'm sorry for my absence. I can't review this PR this week but I can next week.

[Canoming]

Hi, @AlejandroSopena , I've updated the code according to your comments. The Readme.md file is a deprecated 'proof of principle' demonstration for earlier development. I accidentally committed that file to the branch, and it's deleted now. I'm working on a more practical example for the quantum_network, which will be added to a separate PR for documentations later.