Bloch Messiah decomposition from The Walrus

[sduquemesa]

NOTE: This PR is using thewalrus/main. DO NOT MERGE until there is a new release of The Walrus (don't forget to change the requirement files accordingly).

Context: The current implementation of the Bloch-Messiah decomposition fails for degenerate symplectic matrices.

Description of the Change: The Walrus included a new implementation of the Bloch-Messiah decomposition that solves the issue. This PR swaps the SF implementation for the new improved implementation on TW.

Benefits: Bloch-Messiah works for degenerate symplectic matrices.

Possible Drawbacks: None.

Related GitHub Issues: Closes #728

[codecov[bot]]

Codecov Report

Merging #729 (720637e) into master (9a9a352) will decrease coverage by 59.45%. The diff coverage is 66.66%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #729       +/-   ##
===========================================
- Coverage   98.33%   38.88%   -59.46%     
===========================================
  Files          78       78               
  Lines        9603     9578       -25     
===========================================
- Hits         9443     3724     -5719     
- Misses        160     5854     +5694     

Impacted Files	Coverage Δ
strawberryfields/decompositions.py	12.88% <66.66%> (-86.57%)	:arrow_down:
strawberryfields/io/xir_io.py	8.04% <0.00%> (-90.23%)	:arrow_down:
strawberryfields/apps/subgraph.py	10.11% <0.00%> (-89.89%)	:arrow_down:
strawberryfields/apps/clique.py	10.67% <0.00%> (-89.33%)	:arrow_down:
strawberryfields/apps/qchem/dynamics.py	12.65% <0.00%> (-87.35%)	:arrow_down:
strawberryfields/backends/tfbackend/ops.py	11.96% <0.00%> (-86.31%)	:arrow_down:
...fields/backends/gaussianbackend/gaussiancircuit.py	13.02% <0.00%> (-86.21%)	:arrow_down:
strawberryfields/backends/tfbackend/states.py	13.33% <0.00%> (-85.65%)	:arrow_down:
strawberryfields/tdm/utils.py	11.60% <0.00%> (-85.64%)	:arrow_down:
strawberryfields/plot.py	13.15% <0.00%> (-85.09%)	:arrow_down:
... and 49 more

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