Enable connecting multiple list Attributes

[servantftransperfect]

Enable the connection of multiple listAttributes to the same destination listAttribute

[codecov]

Codecov Report

❌ Patch coverage is 87.77778% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.21%. Comparing base (912a71d) to head (35cf5ba).
⚠️ Report is 51 commits behind head on develop.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
meshroom/core/graph.py	68.57%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3000      +/-   ##
===========================================
+ Coverage    82.01%   83.21%   +1.19%     
===========================================
  Files           69       70       +1     
  Lines         9310    10144     +834     
===========================================
+ Hits          7636     8441     +805     
- Misses        1674     1703      +29     

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[cbentejac]

This seems to work correctly, including when performing undos and redos, but some unit tests should definitely be added.

[fabiencastan]

In my opinion, it is not a good idea to create connections between lists and, in some cases, between elements within the lists, without providing the end user with any information. This leads to errors and confusion.

[fabiencastan]

In the short term, I would recommend to create a node with a list<list> in input that creates a flatten list in output.

[fabiencastan]

#3061