Superseded, please use dfMergeVariablesRelevel
dfMergeTwoVariablesRelevel(df = NULL, colnames = "", levelslist = NULL)A data.frame with the variables (columns) to be merged into one vector.
A vector of names of columns in `df` that hold the variables
to be merged, or a selection of columns as per select.
A names list with one slice each for a new value to be used for a vector of old values (optional).