A character specifying which function to use to categorize
samples by signature scores. Can be one of:
"original" (default), apply the original publication method; if
categorization is not expected, the median score is used as a threshold;
"mean"/"median", samples will be classified as "low" or "high" with
respect to the mean/median signature score, respectively;
"tertiles", samples will be classified as "<= T1" (score lower than
first tertile), "(T1, T2]" (score between first and second tertiles),
"> T2" (score higher than second tertile);
"quartiles", samples will be classified as "<= Q1" (score lower than
first quartile), "(Q1, Q2]" (score between first and second quartiles),
"(Q2, Q3]" (score between second and third quartiles), "> Q3" (score
higher than third quartile).