factor level 맞추기
features = colnames(allDD)
for (f in features){
\tif( (class(allDD[[f]]) == "character") || (class(allDD[[f]]) == "factor"))
\t{
\t\tlevels = unique(allDD[[f]])
\t\tallDD[[f]] = factor(allDD[[f]], level = levels)
\t}
} 
Simple Imputation (simputation)
https://cran.r-project.org/web/packages/simputation/simputation.pdf
