Issue #84: Prune not helpful to arg flow maths

[seabbs]

Description

This PR closes #84. It removes some lines of maths that don't seem useful to the reader and weren't in all analytical solutions. It also removes the acknowledgement section from the vignette as we already have this contribution flagged via the all contributors bot.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.29%. Comparing base (7180ddc) to head (8aa738b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #116   +/-   ##
=======================================
  Coverage   99.29%   99.29%           
=======================================
  Files          10       10           
  Lines         427      427           
=======================================
  Hits          424      424           
  Misses          3        3           

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[seabbs]

LGTM (self-review)