Remove IES

[sondreso]

Issue
The Iterative Ensemble Smoother (IES) is sensitive to failing realizations and deactivation of observations. It's usage is not widespread, and in most cases ES-MDA is the preferred alternative. We will remove IES from ERT, but we will keep the implementation of the algorithm in the iterative_ensemble_smoother
repository.

Approach
Remove IES from ERT ✂️

(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 tests/everest -n auto --hypothesis-profile=fast -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 #9843 will improve performances by 14.34%

_{Comparing sondreso:remove-ies (b628585) with main (905fb9a)}

Summary

⚡ 2 improvements
✅ 22 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_load_from_context[gen_x: 20, sum_x: 20 reals: 10]	7.2 ms	6.4 ms	+12.29%
⚡	test_direct_dark_performance_with_storage[gen_x: 20, sum_x: 20 reals: 10-summary-get_record_observations]	1.4 ms	1.2 ms	+14.34%

[oyvindeide]

Nice!