"Model" class, containing a (generic) learning function, which from
data + target [+ params] returns a prediction function X --> y.
Parameters for cross-validation are either provided or estimated.
-Model family can be chosen among "rf", "tree", "ppr" and "knn" for now.
+Model family can be chosen among "tree", "ppr" and "knn" for now.
}
\section{Public fields}{
\if{html}{\out{<div class="r6-fields">}}