[PierreRaybaut]

Fixes #400

User feedback on DataLab 1.3.0 about blackbody signals, curve fitting and synthetic images. The fixes themselves are in Sigima and SigimaX; this PR adapts DataLab's tests and documents the user-facing behavior.

What users get (with the companion PRs)

• Processing > Fitting > Planckian fit now fits blackbody data in physical units. It used to return a curve about 150 times too low, with no warning.
• Exponential fits now work for short time constants whatever the X unit, e.g. a 1 ms time constant with X in seconds.
• Interactive fit dialogs show the formula of the model, and each parameter label shows its symbol in the formula.
• Create > Blackbody (Planck's law) gives a signal with axis titles and units: wavelength in m, spectral radiance in W·sr⁻¹·m⁻³.
• Synthetic images computed over a domain (2D Gaussian, 2D ramp, sinusoidal grating, ring pattern, Siemens star, 2D sinc) are displayed over that domain instead of in pixels. A new Physical coordinates option (enabled by default) controls this. Images created by earlier versions keep their pixel coordinates when their creation parameters are edited or recomputed.

Changes in this PR

• Tests:
  • geometry transform and analysis-recompute tests define ROIs and centroids in pixels, so they now request pixel coordinates explicitly;
  • a new test checks that creation parameters stored by DataLab 1.3, in object metadata and in history entries, recreate images in pixel coordinates.
• Documentation:
  • Fitting: the formula of each model and the name under which each parameter is stored, and how to derive the temperature and emissivity from a Planckian fit;
  • Create signals: blackbody axes;
  • Create images: the "Physical coordinates" option;
  • French translations for all of the above.
• Release notes 1.4.0: new features, changes and bug fixes.

No dependency change: DataLab already requires Sigima ≥ 1.3.0 and SigimaX ≥ 1.1.0.

Companion PRs (merge order: Sigima → SigimaX → DataLab)

• Sigima: Fix Planckian/exponential fits on physical units; add fit formulas Sigima#75
• SigimaX: Show the model formula in interactive fit dialogs SigimaX#10

Validation

• Full DataLab test suite: 1023 passed, 3 skipped, run against the Sigima and SigimaX branches of the same name.
• Ruff passes.