yields groups organized as a list of Div structures from a conjunctive or disjunctive codification
col_groups = groupcreate(xi,classes);
a matrix (n x q) or a Div structure
a conjunctive vector (n x 1) or a disjunctive matrix of dimensions (n x nclasses) or the Div structures corresponding to the previous cases
a vector of Div structures
col_groups(i).d contains the observations of group i