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 29.09091% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.78%. Comparing base (62a4d95) to head (5801446).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
process/core/io/vary_run/config.py	20.45%	35 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   51.78%   51.78%   -0.01%     
==========================================
  Files         150      150              
  Lines       30285    30321      +36     
==========================================
+ Hits        15684    15702      +18     
- Misses      14601    14619      +18     

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[timothy-nunn]

The old files are now not being deleted:

We only want the 2_* files to be left (in this example) as the others a produced by non-convergent input files

[clmould]

The old files are now not being deleted:

We only want the `2_*` files to be left (in this example) as the others a produced by non-convergent input files

@timothy-nunn This is how it's currently working on main

[timothy-nunn]

The old files are now not being deleted:

We only want the 2_* files to be left (in this example) as the others a produced by non-convergent input files

@timothy-nunn This is how it's currently working on main

@clmould the image is not working for me, but that surprises me. I don't think thats how it worked when I last used vary run, and I don't think that is how it should work because the intermediate files are not useful imo. This PR seems like a good opportunity to fix this.

[clmould]

Keeping the intermediate files for now as removing them wasn't as straightforward as initially thought. #4227