Sub state vector

[AntonKast]

The current implementation appears to be exponential in space and time. The present change, due originally to jamieas@google.com, replaces python loops with numpy functions motivated by standard methods for computing partial traces. The change is runnable in Colab, including tests and a performance benchmark, at

https://colab.sandbox.google.com/drive/1qWJTnlT6hPXI8cQsTwyqQveEtn7fGhNc?resourcekey=0-67OmbvgwTlf9fnKp3ppbrw

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.63%. Comparing base (76910aa) to head (c13efc6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8077      +/-   ##
==========================================
- Coverage   99.63%   99.63%   -0.01%     
==========================================
  Files        1110     1110              
  Lines      100067   100075       +8     
==========================================
+ Hits        99705    99712       +7     
- Misses        362      363       +1     

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