Determine the function to be used in the normalization step of evaluate().
normalize_function(method = EnumEvalMethod, norm = NULL)Returns function norm if not NULL.
Otherwise, it determines the result depending on method:
"set":norm_none()
"prob":norm_sum()
"fuzzy":norm_none()
"fuzzynorm":norm_max()
Fails with an error if the result is not an R function.
One of: "set" (default), "prob", "fuzzy" or "fuzzynorm".
Some normalization function of the form function(num_vector), or NULL.
evaluate, norm_none(), norm_max(), norm_sum(),