Add T, S gates

[mlazzarin]

Closes #524. I added S and T, along with their conjugate transpose SDG and TDG. Still need to update the tests.

[codecov]

Codecov Report

Merging #525 (1394fd9) into master (d2c7e1a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #525    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           85        85            
  Lines        12284     12439   +155     
==========================================
+ Hits         12284     12439   +155     

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

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

Impacted Files	Coverage Δ
src/qibo/tests/test_backends_matrices.py	100.00% <ø> (ø)
src/qibo/tests/test_cirq.py	100.00% <ø> (ø)
src/qibo/abstractions/gates.py	100.00% <100.00%> (ø)
src/qibo/backends/matrices.py	100.00% <100.00%> (ø)
src/qibo/core/gates.py	100.00% <100.00%> (ø)
src/qibo/tests/test_abstract_circuit_qasm.py	100.00% <100.00%> (ø)
src/qibo/tests/test_abstract_gates.py	100.00% <100.00%> (ø)
src/qibo/tests/test_core_circuit_qasm.py	100.00% <100.00%> (ø)
src/qibo/tests/test_core_gates.py	100.00% <100.00%> (ø)
src/qibo/tests/test_core_gates_density_matrix.py	100.00% <100.00%> (ø)
... and 1 more

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[mlazzarin]

Ok, I added the tests, now it's ready for review.

[stavros11]

Thanks for implementing this. There is a test file called test_core_circuit_qasm.py where we create circuits from qasm in qibo and cirq and we verify that their execution agrees. Since qasm was the original motivation of this PR, I would suggest to add a test involving these gates there. Other than that, everything looks good.

[mlazzarin]

I implemented your suggestions. Let me know if you agree with the changes.

[stavros11]

Thank you for the updates, looks good to me.

[andrea-pasquale]

Thanks for this implementation.
LGTM.

[AdrianPerezSalinas]

Hi all, I know that you made great work and these gates are functional. However, the .draw() method does not recognize them. I am using the current '0.1.7rc2.dev0' version.