🎨 Add effective charge and ionisation profiles to plot_proc

[chris-ashe]

This pull request standardizes the naming conventions for electric charge-related variables and updates example data files to reflect these changes. The main focus is on replacing the old zeff and zeffai identifiers with new, more descriptive variable names that follow the updated standards. Additionally, documentation is improved to clarify the new conventions.

✨ New variables

• n_charge_plasma_effective_profile
• n_charge_impurity_profile

🔄 Renames

• zeff -> n_charge_plasma_effective_vol_avg
• zeffai -> n_charge_plasma_effective_mass_weighted_vol_avg

📓 Documentation

• Added explicit naming conventions for electric charge variables in documentation/development/standards.md, introducing the charge_ and n_charge_ prefixes for electric charges and charge numbers, respectively.

🎨 Output

These changes ensure consistency and clarity in variable naming, making the codebase easier to maintain and understand.## Description

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 20.00000% with 96 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.01%. Comparing base (b143420) to head (46226ea).

Files with missing lines	Patch %	Lines
process/io/plot_proc.py	4.00%	72 Missing ⚠️
process/physics.py	23.07%	20 Missing ⚠️
process/current_drive.py	50.00%	2 Missing ⚠️
process/output.py	0.00%	1 Missing ⚠️
process/stellarator.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3977      +/-   ##
==========================================
- Coverage   46.09%   46.01%   -0.08%     
==========================================
  Files         123      123              
  Lines       28888    28957      +69     
==========================================
+ Hits        13316    13325       +9     
- Misses      15572    15632      +60     

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[j-a-foster]

Happy with changes. Some of the variable replaces need checking though.

[timothy-nunn]

Whoops, missed the error on two of the regression tests