Draft: Clean-up of LamarrPropagator

The tracking GANs are implemented following a "residual-like" logic that model the outputs as a deviation from a multivariate normal distribution. The logic implementing this deviation was included in LamarrPropagator but a future parametrization of the tracking uncertainties may not rely on the same logic.

I took the opportunity of this clean-up to move this "residual"-like logic in the parametrization itself.

Note: this modification breaks Lamarr if the parametrizations are not updated to the corresponding version.