Column Transformer

Implemented in `column_transformer.R`.

transform=function(df){
  # ...
  if(!is.null(num_pipeline) && length(num_cols) > 0L){
    parts[[length(parts)+1]] <-
      as_feature_matrix(num_pipeline$transform(df[,num_cols,drop=FALSE]))
  }
  # ... same for cat_pipeline ...
  out <- do.call(cbind, parts)
  out
}