DiseasyImmunity: Update default optimisers

[RasmusSkytte]

Intent

This PR introduces a few more quality of life updates to the optimiser script and updates the default optimisers in DiseasyImmunity based on the results of a fresh re-run of optimiser script.

This new run also include a mixed penalty mode which the vignette is updated to display. That is, we test a scenario where monotonous = TRUE while individual_level = FALSE

Approach

N/A

Known issues

N/A

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.19%. Comparing base (62c0545) to head (7edf802).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #289      +/-   ##
==========================================
+ Coverage   67.16%   67.19%   +0.02%     
==========================================
  Files          28       28              
  Lines        4648     4648              
==========================================
+ Hits         3122     3123       +1     
+ Misses       1526     1525       -1     

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