The group classification for each observation as vector. This group
classification can be used for sjc.grpdisc-function to
check the goodness of classification.
The returned vector includes missing values, so it can be appended
to the original data frame data.
References
Maechler M, Rousseeuw P, Struyf A, Hubert M, Hornik K (2014) cluster: Cluster Analysis Basics and Extensions. R package.