Add rule for GPI gate

[andrea-pasquale]

Closes #551.

Adding rule for implementing GPI gate as R(pi) gate with arbitrary azimuthal angle.
I believe that I don't need to do anything in Qibo, right?

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 ✅

Comparison is base (8bb0327) 63.89% compared to head (16e375d) 63.94%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #804      +/-   ##
==========================================
+ Coverage   63.89%   63.94%   +0.04%     
==========================================
  Files          49       49              
  Lines        5792     5799       +7     
==========================================
+ Hits         3701     3708       +7     
  Misses       2091     2091              

Flag	Coverage Δ
unittests	63.94% <100.00%> (+0.04%)	⬆️

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[stavros11]

Thanks @andrea-pasquale. The implementation looks correct to me, but it would be if @igres26 or @renatomello who have more experience with IONQ can also confirm.

We should keep in mind that the matrix representation of this gate has det=-1, unlike U3, GPI2 and others that have det=1, so we may have similar issues to the ones we had with the Hadamard.

[igres26]

I think there is a global phase difference of what we are doing and the explicit matrix. We might want to note that.

[andrea-pasquale]

Thanks for the review @igres26 @stavros11.
I've added a comment (16e375d) with a brief explanation. Let me know if this is ok or if you would like a stronger message like a warning.