Add CSWAP gate

[GabrielePalazzo]

Checklist:

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

This draft aims to address #

In this PR I created a CSWAP gate.
While this automatically fixes the issue #1802, it feels just a copy-paste of the code in gates.py.
In alternative, the qasm parser can detect controlled gates and instantiate them.

[codecov]

Codecov Report

❌ Patch coverage is 47.61905% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.43%. Comparing base (4815b7a) to head (7625bc7).

Files with missing lines	Patch %	Lines
src/qibo/gates/gates.py	44.44%	10 Missing ⚠️
src/qibo/backends/npmatrices.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1803      +/-   ##
==========================================
- Coverage   99.51%   99.43%   -0.09%     
==========================================
  Files          79       79              
  Lines       13274    13295      +21     
==========================================
+ Hits        13210    13220      +10     
- Misses         64       75      +11     

Flag	Coverage Δ
unittests	99.43% <47.61%> (-0.09%)	⬇️

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[renatomello]

I would suggest avoiding creating yet another gate that is controlled by default. This can be avoided by using the controlled_by method of the Gate class.

I am suggesting this because all gates that are controlled by default

qibo/src/qibo/gates/abstract.py

Line 35 in 91fb509

GATES_CONTROLLED_BY_DEFAULT = [

already generate secondary issues as is, e.g. Transpilation issues, gate-counting issues. Since these gates would have to be refactored anyway, it would be better not to increase the number of gates controlled by default.

There are some rules to deal with edge cases in the _openqasm.py file.

qibo/src/qibo/models/_openqasm.py

Line 115 in 91fb509

def _qibo_gate_name(gate):

Oner rule for the CSWAP from qasm strings could be added there to be converted to a qibo SWAP with one control, and the import issue would be solved without having to create another gate class.