analysis of variance et comparison of means by Student-Newman-Keuls (SNK)
res_snk = snk(x,classes);
a matrix (n x q) or a div structure
a vector identifying the classes
res_snk.anova contains the analysis of variance of each of the variables in x
res_snk.snk contains the classification of the means by SNK
Warning: SNK is correct for a same number of observations in each class; if not, SNK gives an approximate result with an error increasing with the variability of the number of observations of the classes