Identifies numeric columns. Stacks values and supplies factor level 'numeric'.
# S3 method for data.frame
numerics(x, ..., na.rm_num = FALSE, all_levels = FALSE)
same class as x
data.frame; names must not include 'name' or 'level'
passed to classifiers
whether to drop NA observations; passed to gather
as na.rm
whether to supply records for unobserved levels