Fix misfit config so it is possible to configure a subset of observations

[oyvindeide]

Issue
Resolves #7211

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 88.33333% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 85.25%. Comparing base (819de94) to head (8466fdf).

Files	Patch %	Lines
...k_implementations/workflows/misfit_preprocessor.py	53.84%	6 Missing ⚠️
...rc/ert/gui/tools/run_analysis/run_analysis_tool.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7263      +/-   ##
==========================================
+ Coverage   85.23%   85.25%   +0.02%     
==========================================
  Files         386      387       +1     
  Lines       23031    23058      +27     
  Branches      874      889      +15     
==========================================
+ Hits        19631    19659      +28     
- Misses       3290     3291       +1     
+ Partials      110      108       -2     

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[sondreso]

LGTM!