Add box-plots to standard deviation plot

[dafeda]

Issue
Resolves #my_issue

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure tests pass locally (after every commit!)

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.77%. Comparing base (04d8d61) to head (4a33565).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ert/gui/plottery/plots/std_dev.py	91.17%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8565      +/-   ##
==========================================
+ Coverage   90.68%   90.77%   +0.08%     
==========================================
  Files         342      342              
  Lines       20898    20966      +68     
==========================================
+ Hits        18952    19032      +80     
+ Misses       1946     1934      -12     

Flag	Coverage Δ
gui-tests	75.84% <91.17%> (-0.03%)	⬇️
integration-tests	53.94% <2.94%> (-0.09%)	⬇️
unit-tests	68.72% <2.94%> (-0.04%)	⬇️

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[xjules]

Superseded by #8833