Unit overhaul 4/6: Separate unit into x_unit/y_unit and update tutorials

[henrikjacobsenfys]

Replace the single unit on EasyDynamicsModelBase with independent x_unit (axis, default meV) and y_unit (output, default dimensionless), and migrate every subclass in the hierarchy: components, diffusion models, convolution, instrument/resolution/sample/background models, experiment and analysis. convert_unit becomes convert_x_unit/convert_y_unit, implemented via a shared rollback helper, and evaluate gains an output='numpy'|'scipp' selector. Also adds the additive get_fit_targets() model API (consumed by #226).

The base-class rename is irreducibly atomic — it forces all subclasses to change together, so this core can't be split further without a temporary compatibility shim.

Tutorial-notebook updates are distributed across the stack so they run green on every PR, not just at the top — each PR's tutorial-tests job executes all notebooks against that branch. This PR carries only the x_unit/y_unit notebook edits (sample_model and tutorial0_more_advanced); the FitBinding(targets=…) notebook changes land in #226 and the ExpressionComponent ones in #227.

Two independent changes that were previously bundled into this PR have been carved into stacked follow-ups: the FitBinding/FitTarget redesign (#226) and the ExpressionComponent physical-constants/parameter-units feature (#227).

All unit tests pass.

---

Stack — splitting the monolithic "Unit system overhaul" (#217) into a reviewable series. Merge bottom-up:

1. Add unit-conversion utilities and FitTarget → develop (Unit overhaul 1/6: Add unit-conversion utilities and FitTarget #221)
2. Centralize Q-index validation via verify_Q_index → split/01-unit-utils (Unit overhaul 2/6: Centralize Q-index validation via verify_Q_index #222)
3. Store Q as a scipp Variable in 1/angstrom → split/02a-verify-q-index (Unit overhaul 3/6: Store Q as a scipp Variable in 1/angstrom #223)
4. Separate model unit into x_unit/y_unit + tutorials → split/02b-q-variable (Unit overhaul 4/6: Separate unit into x_unit/y_unit and update tutorials #224)
5. Redesign FitBinding around FitTargets → split/02c-unit-rename (Unit overhaul 5/6: Redesign FitBinding around FitTargets #226)
6. ExpressionComponent physical constants + parameter units → split/02d-fit-binding (Unit overhaul 6/6: ExpressionComponent physical constants and parameter units #227)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 99.48454% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.66%. Comparing base (2379d58) to head (2aa65b8).

Files with missing lines	Patch %	Lines
src/easydynamics/analysis/analysis1d.py	96.00%	0 Missing and 1 partial ⚠️
src/easydynamics/sample_model/components/mixins.py	87.50%	0 Missing and 1 partial ⚠️
src/easydynamics/sample_model/sample_model.py	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##           split/02b-q-variable     #224      +/-   ##
========================================================
+ Coverage                 97.47%   98.66%   +1.18%     
========================================================
  Files                        53       53              
  Lines                      3687     3892     +205     
  Branches                    640      664      +24     
========================================================
+ Hits                       3594     3840     +246     
+ Misses                       62       29      -33     
+ Partials                     31       23       -8     

Flag	Coverage Δ
unittests	98.66% <99.48%> (+1.18%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/easydynamics/analysis/analysis.py	96.41% <100.00%> (+1.34%)	⬆️
...asydynamics/base_classes/easydynamics_modelbase.py	100.00% <100.00%> (ø)
...easydynamics/convolution/analytical_convolution.py	98.76% <ø> (ø)
src/easydynamics/convolution/convolution.py	100.00% <100.00%> (ø)
src/easydynamics/convolution/convolution_base.py	100.00% <100.00%> (ø)
.../easydynamics/convolution/numerical_convolution.py	100.00% <100.00%> (+4.16%)	⬆️
...dynamics/convolution/numerical_convolution_base.py	97.56% <100.00%> (+0.19%)	⬆️
src/easydynamics/experiment/experiment.py	98.88% <100.00%> (+1.22%)	⬆️
src/easydynamics/sample_model/background_model.py	100.00% <ø> (ø)
.../easydynamics/sample_model/component_collection.py	100.00% <100.00%> (+0.86%)	⬆️
... and 20 more

... and 3 files with indirect coverage changes

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