[FIX] #226 Using reference comparison with literal values

[Abelarm]

Fixing the #226

[codecov]

Codecov Report

Merging #230 (b27e7d8) into develop (b8133b9) will not change coverage.
The diff coverage is 36.36%.

@@           Coverage Diff            @@
##           develop     #230   +/-   ##
========================================
  Coverage    45.76%   45.76%           
========================================
  Files           84       84           
  Lines         9096     9096           
========================================
  Hits          4163     4163           
  Misses        4933     4933           

Impacted Files	Coverage Δ
solcore/analytic_solar_cells/QE.py	6.18% <0.00%> (ø)
solcore/poisson_drift_diffusion/QWunit.py	14.35% <0.00%> (ø)
solcore/quantum_mechanics/kp_QW.py	4.50% <0.00%> (ø)
solcore/units_system/units_system.py	73.54% <ø> (ø)
solcore/absorption_calculator/absorption_QW.py	79.53% <100.00%> (ø)
solcore/quantum_mechanics/high_level_kp_QW.py	33.82% <100.00%> (ø)

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