Added fixed_atoms array

[cpignedoli]

Needed to have fixed_atoms in quantumespresso calculations with the AiiDAlab QE app.
connected to:
aiidalab/aiidalab-qe#1397

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.25%. Comparing base (cc83440) to head (9f5b00e).

Files with missing lines	Patch %	Lines
aiidalab_widgets_base/structures.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
- Coverage   84.29%   84.25%   -0.04%     
==========================================
  Files          18       18              
  Lines        3584     3589       +5     
==========================================
+ Hits         3021     3024       +3     
- Misses        563      565       +2     

Flag	Coverage Δ
python-3.11	84.25% <66.66%> (-0.04%)	⬇️
python-3.9	84.26% <66.66%> (-0.04%)	⬇️

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