Add environment flag for default qibo backend

[stavros11]

Fixes #609.

[codecov]

Codecov Report

Merging #613 (dde9de4) into abstractions (c4b7aaf) will decrease coverage by 0.00%.
The diff coverage is 92.10%.

@@               Coverage Diff                @@
##           abstractions     #613      +/-   ##
================================================
- Coverage         96.99%   96.99%   -0.01%     
================================================
  Files                74       74              
  Lines             10533    10533              
================================================
- Hits              10217    10216       -1     
- Misses              316      317       +1     

Flag	Coverage Δ
unittests	96.99% <92.10%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/qibo/backends/abstract.py	90.00% <ø> (ø)
src/qibo/backends/__init__.py	69.33% <78.57%> (+0.31%)	⬆️
src/qibo/backends/numpy.py	92.00% <100.00%> (-0.08%)	⬇️
src/qibo/backends/tensorflow.py	98.87% <100.00%> (-0.04%)	⬇️
src/qibo/hamiltonians/hamiltonians.py	98.36% <100.00%> (+0.02%)	⬆️

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