Add support for multiple occurrences of design matrix

[xjules]

Issue
Resolves #9532

Approach
All occurrences of design matrices will be merged into one upon initialization.

(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 (7774b35) with main (4480295)}

Summary

✅ 24 untouched benchmarks

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.73%. Comparing base (4480295) to head (7774b35).

Files with missing lines	Patch %	Lines
src/ert/config/design_matrix.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9583   +/-   ##
=======================================
  Coverage   91.73%   91.73%           
=======================================
  Files         426      426           
  Lines       26542    26567   +25     
=======================================
+ Hits        24348    24372   +24     
- Misses       2194     2195    +1     

Flag	Coverage Δ
cli-tests	39.82% <80.76%> (+0.04%)	⬆️
everest-models-test	34.12% <19.23%> (-0.02%)	⬇️
gui-tests	74.28% <19.23%> (-0.07%)	⬇️
integration-test	37.95% <19.23%> (-0.01%)	⬇️
performance-tests	51.54% <19.23%> (-0.04%)	⬇️
test	39.47% <19.23%> (-0.03%)	⬇️
unit-tests	74.23% <88.46%> (+0.01%)	⬆️

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[larsevj]

Is it due to mypy that we go via pc_self instead of just appending to self.parameter_configuration?

[larsevj]

🚀