Fixed a bug in std dev plot where color gradiant range was always 0 to 1 & two corner cells where always white

[eivindjahren]

These images were produces by using the heat equation as forward model in ert.

Although it's hard to see whether the scaling is better or not, the new code seems to have fixed a strange where two corner cells are completely white. Also, the first row and column look strangely offset in the old code.

• 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 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.87%. Comparing base (f760d4c) to head (bb78072).

Files	Patch %	Lines
src/ert/gui/plottery/plots/std_dev.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #8408       +/-   ##
===========================================
+ Coverage   69.76%   90.87%   +21.10%     
===========================================
  Files         347      347               
  Lines       21066    21077       +11     
===========================================
+ Hits        14696    19153     +4457     
+ Misses       6370     1924     -4446     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.