Backend stack and broadcast to

[apchytr]

Context: Implementing backend methods for future use. E.g. triples batches and new PolyExpAnsatz

Merge before: Batching triples.py and symplectics.py

Description of the Change: Implement stack and broadcast_to in the backend. Also cleaned up math.allclose

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.17%. Comparing base (e17bc9b) to head (56d43a2).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #561   +/-   ##
========================================
  Coverage    90.17%   90.17%           
========================================
  Files          102      102           
  Lines         6175     6180    +5     
========================================
+ Hits          5568     5573    +5     
  Misses         607      607           

Files with missing lines	Coverage Δ
mrmustard/math/backend_manager.py	98.25% <100.00%> (+0.02%)	⬆️
mrmustard/math/backend_numpy.py	100.00% <ø> (ø)
mrmustard/math/backend_tensorflow.py	100.00% <ø> (ø)

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[aplund]

This all looks fine.