Refactor gui node to use dataclasses

[jonathan-eq]

Issue Resolves #7382

Approach The commit in this PR refactors the nodes in GUI (RootNode, IterNode, RealNode, ForwardModelNode) to use explicit dataclasses for each type instead of having the type as a field.

(Screenshot of new behavior in GUI if applicable)

• [x] PR title captures the intent of the changes, and is fitting for release notes.
• [x] Added appropriate release note label
• [x] 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 88.81579% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 85.35%. Comparing base (6aa3798) to head (da3164f). Report is 11 commits behind head on main.

Files	Patch %	Lines
src/ert/gui/model/node.py	89.41%	9 Missing :warning:
src/ert/gui/model/snapshot.py	87.30%	8 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7862      +/-   ##
==========================================
- Coverage   85.39%   85.35%   -0.05%     
==========================================
  Files         381      381              
  Lines       23593    23705     +112     
  Branches      883      879       -4     
==========================================
+ Hits        20147    20233      +86     
- Misses       3339     3365      +26     
  Partials      107      107              

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[jonathan-eq]

Codecov Report

Attention: Patch coverage is 87.40741% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 85.43%. Comparing base (5371acb) to head (e586eff). Report is 2 commits behind head on main.

Files Patch % Lines src/ert/gui/model/node.py 88.15% 9 Missing ⚠️ src/ert/gui/model/snapshot.py 86.44% 8 Missing ⚠️ Additional details and impacted files ☔ View full report in Codecov by Sentry. 📢 Have feedback on the report? Share it here.

I am not getting CodeCov to work. I cannot see what lines are not covered.

[berland]

Dataclasses do look nicer!