Get measured data using active realizations

[dafeda]

This bug caused misfit_preprocessor to fail on cases with failed realizations.

(cherry picked from commit 0b8b74a)

Pre review checklist

• Read through the code changes carefully after finishing work
• Make sure tests pass locally (after every commit!)
• Prepare changes in small commits for more convenient review (optional)
• PR title captures the intent of the changes, and is fitting for release notes.
• Updated documentation
• Ensured that unit tests are added for all new behavior (See
  Ground Rules),
  and changes to existing code have good test coverage.

Pre merge checklist

• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.

[codecov-commenter]

Codecov Report

Merging #6188 (e3e6551) into version-6.0 (33178ef) will decrease coverage by 0.01%.
The diff coverage is 83.33%.

@@               Coverage Diff               @@
##           version-6.0    #6188      +/-   ##
===============================================
- Coverage        81.79%   81.79%   -0.01%     
===============================================
  Files              351      351              
  Lines            21843    21847       +4     
  Branches           758      758              
===============================================
+ Hits             17867    17870       +3     
  Misses            3724     3724              
- Partials           252      253       +1     

Files	Coverage Δ
src/ert/data/_measured_data.py	78.57% <83.33%> (-0.16%)	⬇️

... and 4 files with indirect coverage changes

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