varyrun stop IN.DAT being created

[clmould]

Description

Closes #4204
Stops creating IN.DAT, MFILE.DAT and OUT.DAT in VaryRun runs (so now only makes the 0_, 1_ etc), update the __next__ method so no files are created once the max iterations are reached (as before if it reached max iterations it would create the next IN.DAT then error saying it can't find the associated MFILE), add messages to make clear if that iteration's file converged or not, update docs and varyrun example

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 33.33333% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.08%. Comparing base (b9e97d7) to head (976c7e5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
process/core/io/vary_run/config.py	25.58%	32 Missing ⚠️
process/core/io/vary_run/tools.py	62.50%	3 Missing ⚠️
process/main.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4206      +/-   ##
==========================================
- Coverage   52.10%   52.08%   -0.02%     
==========================================
  Files         148      148              
  Lines       30389    30414      +25     
==========================================
+ Hits        15835    15842       +7     
- Misses      14554    14572      +18     

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