Simplify PlotLimits

[eivindjahren]

The PlotLimits class implements runtime typechecking in a very magical way. The type checking isn't really used for anything so it is more useful to implement it in a such a way that it can be type checked by mypy.

All functionality of the "undostack" setting the plotlimits in the custmization panel seems to still work as expected:

The DEPTH_AXIS seems to be unused, and index axis makes the corresponding customization field not accept "-" so the minimum is enforced by the text field:

• [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.
• [x] 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 98.70130% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.53%. Comparing base (37c4d20) to head (01b6c3e). Report is 1 commits behind head on main.

Files	Patch %	Lines
.../tools/plot/customize/limits_customization_view.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8132 +/- ## ========================================== + Coverage 86.51% 86.53% +0.02% ========================================== Files 383 383 Lines 23742 23714 -28 Branches 629 634 +5 ========================================== - Hits 20540 20522 -18 + Misses 3126 3117 -9 + Partials 76 75 -1 ```

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