Full state callback

[stavros11]

Implements a callback that keeps track of the full state vector or density matrix during circuit simulation. This was requested by @igres26 in #555. Usage is similar to existing callbacks, one can add this to a circuit, in the place which a stamp of the state is required, using a CallbackGate.

This PR is complete, I just mark it as do not merge because the test will change if we merge qiboteam/qibojit#74.

[codecov]

Codecov Report

Merging #559 (348c2f3) into master (e2201c7) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #559   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           84        84           
  Lines        12651     12681   +30     
=========================================
+ Hits         12651     12681   +30     

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

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Impacted Files	Coverage Δ
src/qibo/abstractions/callbacks.py	100.00% <100.00%> (ø)
src/qibo/core/callbacks.py	100.00% <100.00%> (ø)
src/qibo/tests/test_core_callbacks.py	100.00% <100.00%> (ø)

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[stavros11]

@scarrazza I updated the test after the latest merge in qibojit. This should be good to go.

[scarrazza]

Thanks!