Construct FSGD Class name from group and non-continuous covariate columns.
get.subject.class(demographics_df, row_idx, class_columns, collapse = "_")data.frame, as returned by read.md.demographics or created manually. Note that the data.frame must not contain any character columns, they should be converted to factors.
integer, the row in the df that belongs to this subject
the column names to use
character string, the separator
character string, the Class name for this subject, derived from the values in the 'class_columns'.