Check for consistent use of dataset_.descriptive.mean vs. bayestopt_.mean_varobs in smoother and estimation

Currently, both structures are still present. prior_posterior_statistics.m sets

bayestopt_.mean_varobs = dataset.descriptive.mean';

to have both. Thus, use should be made consistent and storage be confined to one structure. As the mean of the data belongs to the dataset and not the estimation, the dataset_ structure is better suited than bayestopt_.