split.default doesn't seem to do the right thing for an array
# S4 method for DataLayer
split(x, f, drop = FALSE, ...)vector or data frame containing values to be divided into groups.
a ‘factor’ in the sense that
as.factor(f) defines the grouping, or a
list of such factors in which case their interaction is
used for the grouping.
logical indicating if levels that do not
occur should be dropped (if f is a factor
or a list).
further potential arguments passed to methods.