Add nshot switcher

[scarrazza]

Adds threshold on nshots for selecting the measurement frequency algorithm.

[codecov]

Codecov Report

Merging #341 (17ae569) into metropolisboost (42768c0) will not change coverage.
The diff coverage is 100.00%.

@@                Coverage Diff                @@
##           metropolisboost      #341   +/-   ##
=================================================
  Coverage           100.00%   100.00%           
=================================================
  Files                   75        75           
  Lines                12212     12214    +2     
=================================================
+ Hits                 12212     12214    +2     

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
...o/tests_new/test_measurement_gate_probabilistic.py	100.00% <ø> (ø)
src/qibo/backends/tensorflow.py	100.00% <100.00%> (ø)
src/qibo/config.py	100.00% <100.00%> (ø)

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[stavros11]

Looks good, thanks.