Delegate CircuitResult.probabilities() to the backend

[stavros11]

Moves the implementation of CircuitResult.probabilities() to the backend, to allow having alternative implementations of the probability calculation on hardware backends. Unlike simulation, hardware backends do not rely on the state vector tensor to calculate measurement probabilities.

Similar refactoring may be required for CircuitResult.samples() and CircuitResult.frequencies() depending on hardware needs.

[codecov]

Codecov Report

Merging #618 (487c8fb) into refqibolab (a7257e3) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           refqibolab      #618   +/-   ##
============================================
  Coverage      100.00%   100.00%           
============================================
  Files              77        77           
  Lines           10345     10348    +3     
============================================
+ Hits            10345     10348    +3     

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

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Impacted Files	Coverage Δ
src/qibo/backends/abstract.py	100.00% <100.00%> (ø)
src/qibo/backends/numpy.py	100.00% <100.00%> (ø)
src/qibo/states.py	100.00% <100.00%> (ø)

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