The output of the mcnode for multivariates nodes.
May be "each" (default) if output should be provided for each
variates considered independently, "none" for no output or a vector
of name of function(s) (as a character string) that will be applied
on the variates dimension before any output (ex: "mean",
"median", c("min","max")). The function should have no
other arguments and send one value per vector of values (ex. do not
use "range").