rfe doesn't save recipe performance data

[topepo]

Currently:

modelPred <-
  rfeControl$functions$pred(fitObject, test_x[, retained, drop = FALSE])
if (is.data.frame(modelPred) | is.matrix(modelPred)) {
  if (is.matrix(modelPred)) {
    modelPred <- as.data.frame(modelPred)
    ## in the case where the function returns a matrix with a single column
    ## make sure that it is named pred
    if (ncol(modelPred) == 1)
      names(modelPred) <- "pred"
  }
  modelPred$obs <- test_y
  modelPred$Variables <- sizeValues[k]
} else
  modelPred <-
    data.frame(pred = modelPred,
               obs = test_y,
               Variables = sizeValues[k])

This should also merge in the perf_data df before saving. Or, do this outside of the workflow since the data is the same for each variable subset (within a resample)