4198 raise exceptions on negative fuel ion charge density and temperature profile elements

[jonmaddock]

Both of these unphysical possibilities are always eventually fatal during model evaluation, but were previously confusing due to only causing indirect downstream errors (e.g. in bootstrap current calculation). Now they raise exceptions at source.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.15%. Comparing base (9af55b4) to head (83985ef).

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4217      +/-   ##
==========================================
- Coverage   52.15%   52.15%   -0.01%     
==========================================
  Files         148      148              
  Lines       30431    30435       +4     
==========================================
+ Hits        15870    15872       +2     
- Misses      14561    14563       +2     

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