Add phase_encoder to models.encodings and improve documentation of unary encoder(s)

[renatomello]

Checklist:

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

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (2bfd6ec) 100.00% compared to head (2e91818) 99.99%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
src/qibo/models/encodings.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master    #1190      +/-   ##
===========================================
- Coverage   100.00%   99.99%   -0.01%     
===========================================
  Files           69       69              
  Lines        10107    10124      +17     
===========================================
+ Hits         10107    10123      +16     
- Misses           0        1       +1     

Flag	Coverage Δ
unittests	99.99% <94.11%> (-0.01%)	⬇️

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[Simone-Bordoni]

Thank you for adding this feature, it may be very useful for QML applications. (just fix coverage before merging)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1bf0c4d) 100.00% compared to head (5dee64b) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1190   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           68        68           
  Lines        10186     10203   +17     
=========================================
+ Hits         10186     10203   +17     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

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[BrunoLiegiBastonLiegi]

Thanks, this is very helpful indeed! I would just make the figures a tad smaller if possible.