[grmtrkngtn]

Description

This PR removes two duplicated impurity-radiation data structures:

f_nd_impurity_electron_array is removed in favour of the existing canonical f_nd_impurity_electrons array.

impurity_arr_label is removed in favour of the existing imp_label array.

All impurity iteration variables, radiation calculations, lookups and output now use the canonical arrays directly.

The active impurity species continue to be determined from the non-zero values in f_nd_impurity_electrons, so no separate active-impurity label state is required.

closes #4636

[codecov-commenter]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.24%. Comparing base (0b69cfc) to head (f443f8c).

Files with missing lines	Patch %	Lines
process/models/physics/impurity_radiation.py	50.00%	2 Missing ⚠️
process/models/physics/physics.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4708      +/-   ##
==========================================
- Coverage   50.26%   50.24%   -0.02%     
==========================================
  Files         181      181              
  Lines       30629    30615      -14     
==========================================
- Hits        15395    15384      -11     
+ Misses      15234    15231       -3     

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