Add support for multiple occurrences of design matrix

[xjules]

Issue
Resolves #9532

Approach
todo

(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 unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

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

[codspeed-hq]

CodSpeed Performance Report

Merging #9583 will not alter performance

_{Comparing xjules:mult_dm (8fc6417) with main (e32c0ad)}

Summary

✅ 24 untouched benchmarks

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.48148% with 5 lines in your changes missing coverage. Please review.

Project coverage is 91.88%. Comparing base (01156bb) to head (6c7240e).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ert/config/design_matrix.py	75.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9583      +/-   ##
==========================================
+ Coverage   91.85%   91.88%   +0.02%     
==========================================
  Files         433      433              
  Lines       26768    26804      +36     
==========================================
+ Hits        24587    24628      +41     
+ Misses       2181     2176       -5     

Flag	Coverage Δ
cli-tests	39.79% <81.48%> (+0.03%)	⬆️
everest-models-test	34.55% <18.51%> (-0.04%)	⬇️
gui-tests	72.11% <18.51%> (-0.01%)	⬇️
integration-test	37.18% <18.51%> (-0.02%)	⬇️
performance-tests	52.01% <18.51%> (+0.07%)	⬆️
test	40.65% <18.51%> (+0.20%)	⬆️
unit-tests	74.12% <25.92%> (-0.06%)	⬇️

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[larsevj]

Are these style changes intentional?

[xjules]

Now it becomes a list of lists, so yes it is intentional 😄

[larsevj]

Could we check that the parameter configuration is as expected here?

[larsevj]

Also check/assert that the contents of the df are as expected?