Issue #431: Remove observation process function

[athowes]

Description

This PR closes #431.

It moves the mutating calls into the tests, vignettes, and sticker. It also slims them down to what is needed.

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.
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[athowes]

I think this should be good now.

[athowes]

Failing here but it already has tolerance = 0.2 so should be within range..

[seabbs]

I think tolerance is a relative measure more than you think and so needs maybe some tuning

[seabbs]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.09%. Comparing base (e840b4b) to head (fd1a9f2).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #439      +/-   ##
==========================================
- Coverage   94.24%   94.09%   -0.16%     
==========================================
  Files          16       15       -1     
  Lines         469      457      -12     
==========================================
- Hits          442      430      -12     
  Misses         27       27              

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[athowes]

If we get this merged I can work on #430.