adds stable BS implementation and tests

[ziofil]

Context:
The beamsplitter in the Fock representation suffers from numerical instabilities even for moderate cutoffs (~40)

Description of the Change:
This ports the stable BS implementation from MrMustard, which is just as fast as the unstable BS and offers numerical stability up to arbitrary cutoffs (or at least I ran out of memory before I could notice any).

Benefits:
BS can now be applied to high photon number inputs and outputs. It is as fast as the unstable implementation:

But stable:

Possible Drawbacks:
None

Related GitHub Issues:
None

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (12633ca) to head (5f2ad94).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #405   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           28        28           
  Lines         1922      1935   +13     
=========================================
+ Hits          1922      1935   +13     

Files with missing lines	Coverage Δ
thewalrus/fock_gradients.py	100.00% <100.00%> (ø)

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[nquesada]

You might aswell remove the old one, no?

[ziofil]

Do you want to keep the cutoff as an integer? the new implementation supports any shape.

[ziofil]

@nquesada regarding the cutoff argument, I preserved the old API and made it possible to pass the full shape.