Possibility to extract performance metrics per cross-validation fold

[wardfont]

At the moment, mean and sd of each performance metric is returned based on its values across cross-validation folds. It would be useful for meta-analysis to be able to extract the individual performance metrics for each individual fold. For the tune_gbm function this would mean filtering eval.partial for the best fit and adding it to the results of the function when the user specifies an option to hold on to this info, similar to the keep.fold.fit in the gbm.step function of the dismo package