Selectable minimizers: Fumili, Levenberg-Marquardt and VarPro for the fit models (#387)

## Summary

Makes the minimizer behind `FitModel` selectable and adds two built-in,
dependency-free solvers next to Minuit2's Migrad and Fumili.
`Minimizer.Migrad` stays the default, so existing fits behave as before.

- **`Minimizer` enum** (C++ `FitModel::SetMinimizer`, Python `minimizer`
constructor argument and property): `Migrad`, `Fumili`,
`LevenbergMarquardt`, `VarPro`.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Khalil Ferjaoui <khalilferjaoui@yahoo.fr>

python/aare/__init__.py

@@ -40,6 +40,7 @@ from .Cluster import Cluster
 
 from ._aare import Gaussian, RisingScurve, FallingScurve, Pol1, Pol2, GaussianErfcPlateau, GaussianChargeSharing, GaussianChargeSharingKb
 from ._aare import fit
+from ._aare import Minimizer
 from ._aare import Interpolator
 from ._aare import calculate_eta2, calculate_eta3, calculate_cross_eta3, calculate_full_eta2
 from ._aare import reduce_to_2x2, reduce_to_3x3