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:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

```diff @@ 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](https://app.codecov.io/gh/qiboteam/qibolab/pull/804/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/qiboteam/qibolab/pull/804/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `63.94% <100.00%> (+0.04%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam#carryforward-flags-in-the-pull-request-comment) to find out more.

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[andrea-pasquale]

Thanks for the review @igres26 @stavros11. I've added a comment (https://github.com/qiboteam/qibolab/pull/804/commits/16e375db096b83ca670b982ce3809afa0ddcc55e) with a brief explanation. Let me know if this is ok or if you would like a stronger message like a warning.