Improved BRDF uncertainty propagation

[oceancolorcoder]

Currently the BRDF uncertainty derives from the model uncertainty as described in https://www.eumetsat.int/media/50721. In the future, we should also add BRDF uncertainty associated with the sensitivity to the input variables, i.e., Monte Carlo simulations in punpy.

[oceancolorcoder]

Based on 10.1109/LGRS.2024.3350928, it was concluded that the sensitivity of BRDF to uncertainty in the input parameters is negligible compared to uncertainty across BRDF models. Therefore, no MC runs deemed necessary for improved estimation of BRDF uncertainty.