Add models.encodings.binary_encoder for real-valued data

[renatomello]

Will generalize to complex-valued data in a separate PR soon.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.68%. Comparing base (2e0450b) to head (ffdc2d4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1551   +/-   ##
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  Coverage   99.67%   99.68%           
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  Files          76       76           
  Lines       11234    11271   +37     
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+ Hits        11198    11235   +37     
  Misses         36       36           

Flag	Coverage Δ
unittests	99.68% <100.00%> (+<0.01%)	⬆️

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[MatteoRobbiati]

Could you share the reference/the idea you followed here to implement the binary encoding on this way?
Is that an alternative way to implement the "standard" binary encoding by means of controlled rotations?

[renatomello]

I'm not sure what you mean by "standard", since there are many ways to do binary encoding. This specific one is something that @GoGoKo699 and I have been working on, but it is not published yet. The only difference here is that the amplitudes are parameterized in a specific coordinate system in a $d$-dimensional hypersphere (in this case, called Hopf coordinates).

[BrunoLiegiBastonLiegi]

Thanks, just remove the two commented lines pointed out by @MatteoRobbiati and this can be merged by my side.