R/metrics.R
    measure_icc.RdMetrics are functions that tell how much information would be
lost for a given reduction in the data. reduce.  as_measure() is a
helper function to create new metrics to be used in partitioners.
partitioners can be created with as_partitioner().
measure_icc() assesses information loss by calculating the
intraclass correlation coefficient for the target variables.
measure_icc(.partition_step)a partition_step object
Other metrics:
as_measure(),
measure_min_icc(),
measure_min_r2(),
measure_std_mutualinfo(),
measure_variance_explained()