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 54.54545% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.19%. Comparing base (cc83440) to head (ce9a87b).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
aiidalab_widgets_base/structures.py	54.54%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
- Coverage   84.29%   84.19%   -0.10%     
==========================================
  Files          18       18              
  Lines        3584     3594      +10     
==========================================
+ Hits         3021     3026       +5     
- Misses        563      568       +5     

Flag	Coverage Δ
python-3.11	84.19% <54.54%> (-0.10%)	⬇️
python-3.9	84.20% <54.54%> (-0.10%)	⬇️

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