Feature Selection with Regularized Random Forest. This
package is based on the 'randomForest' package by Andy Liaw.
The key difference is the RRF() function that builds a
regularized random forest. Fortran original by Leo Breiman
and Adele Cutler, R port by Andy Liaw and Matthew Wiener,
Regularized random forest for classification by Houtao Deng,
Regularized random forest for regression by Xin Guan.
Reference: Houtao Deng (2013)