Varyrun fixes

[clmould]

Description

Closes #4058
Also fixes varyrun to vary iteration variable values within factor * initial value, instead of updating each iteration
Also small fix to get value of f_nd_impurity_electrons(13) from the in_dat.data instead of from its default value

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 42.85714% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.83%. Comparing base (bcd37bc) to head (37c6fe9).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
process/core/io/in_dat/base.py	60.00%	2 Missing ⚠️
process/core/io/vary_run/config.py	0.00%	1 Missing ⚠️
process/core/io/vary_run/tools.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4404      +/-   ##
==========================================
+ Coverage   48.80%   48.83%   +0.02%     
==========================================
  Files         151      151              
  Lines       29348    29400      +52     
==========================================
+ Hits        14323    14357      +34     
- Misses      15025    15043      +18     

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