MM einsum

[ziofil]

This PR adds the mm_einsum function, which is like numpy's einsum but for circuit components. It returns the ansatz of the contraction of the components. It requires users to specify the connectivity of the set of components and it calculates the optimal contraction path. TODO: add a heuristic contraction path finding, in case there are many components (the optimal path finding scales factorially with the number of wires).

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 90.01%. Comparing base (2de68b2) to head (e09b030).

Files with missing lines	Patch %	Lines
mrmustard/physics/mm_einsum.py	90.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #544      +/-   ##
===========================================
- Coverage    90.01%   90.01%   -0.01%     
===========================================
  Files          102      103       +1     
  Lines         6222     6302      +80     
===========================================
+ Hits          5601     5673      +72     
- Misses         621      629       +8     

Files with missing lines	Coverage Δ
mrmustard/physics/mm_einsum.py	90.00% <90.00%> (ø)

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