Constructor for admix object
an admix object which is a list with 6 elements: K: number of clusters estimated by ADMIXTURE
nsamples: number of samples used
nmarkers: number of markers used
Q_df: a data.frame of cluster membership probabilities
P_df: a data.frame of estimated marker frequencies in each inferred population
log_info: a data.frame containing the K, CVerror and logLik of the last model.