Functions are passed into the post-processor as a named list. The name
f of the list entry is the function to be applied via
base::do.call.
The list entry itself is another named list, specifying the arguments
to the function f as named arguments.
The functions must take a matrix or data.frame as first argument and
return another matrix or data.frame of the same dimensions as
single output.
Examples of post-processing steps are truncation
(process_truncate_by_iqr,
process_truncate_by_threshold) or
centering / standardizing data (via scale,
see example section below).
Can be useful to apply on simulated datasets, even outside of the
simulation function (e.g. when standardization is only required at the
modeling step).