Batch Refactor

[apchytr]

Context: Currently in develop, there is no interface explicitly defined for the handling of batch dims. This makes things such as adding new batch dimensions (e.g. for measurement outcomes) difficult. Additionally, we wish to introduce the concept of "batch labels" where ideally some class is responsible for the handling of batch labels. As such, in this PR we introduce the Batch class.

Description of the Change: Introduce the Batch class.

Benefits: Having a concrete interface means a cleaner, safer implementation of batching where we can now define arbitrary batch dims and attribute batch labels to them (for mrm einsum).

TODO:
- ArrayAnsatz
- Everything working w arbitrary batch dims
- Handling of batch labels
- Improve ufunc as much as possible
- Cleanup
- Tests

[codecov]

Codecov Report

Attention: Patch coverage is 83.67347% with 16 lines in your changes missing coverage. Please review.

Project coverage is 89.65%. Comparing base (f3c226d) to head (a389643).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
mrmustard/physics/batches.py	77.14%	16 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #521      +/-   ##
===========================================
- Coverage    89.81%   89.65%   -0.17%     
===========================================
  Files           93       94       +1     
  Lines         6098     6158      +60     
===========================================
+ Hits          5477     5521      +44     
- Misses         621      637      +16     

Files with missing lines	Coverage Δ
mrmustard/lab_dev/transformations/base.py	99.42% <100.00%> (ø)
mrmustard/math/backend_manager.py	98.16% <ø> (-0.01%)	⬇️
mrmustard/physics/ansatz/polyexp_ansatz.py	98.55% <100.00%> (-0.05%)	⬇️
mrmustard/physics/batches.py	77.14% <77.14%> (ø)

... and 1 file with indirect coverage changes

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