Add interventions vignette (and remove intvn_reduce_mean argument)

[sbfnk]

• Please check if the PR fulfills these requirements

• I have read the CONTRIBUTING guidelines
• A new item has been added to NEWS.md

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Adds a vignette on how to model interventions.

• What is the current behavior? (You can also link to an open issue here)

n/a

• What is the new behavior (if this is a feature change)?

n/a

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Yes with respect to the previous development version, as it removes the intvn_reduce_mean argument.

• Other information:

Closes #103

[jamesmbaazam]

Thanks for the PR, @sbfnk.

The deletions in the functions all LGTM.

The vignette is really insightful and highlights how to use the models with extra customizations through external functions suited to the user's use case.

I've added a few suggestions here and there.

[codecov-commenter]

Codecov Report

Merging #106 (673b151) into main (5a59132) will decrease coverage by 0.09%.
Report is 21 commits behind head on main.
The diff coverage is n/a.

❗ Current head 673b151 differs from pull request most recent head e614d98. Consider uploading reports for the commit e614d98 to get more accurate results

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
- Coverage   98.91%   98.82%   -0.09%     
==========================================
  Files           9        8       -1     
  Lines         459      424      -35     
==========================================
- Hits          454      419      -35     
  Misses          5        5              

Files	Coverage Δ
R/simulate.r	98.83% <ø> (-0.16%)	⬇️

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