Apply variable labels to data frame
Source:R/set_derived_variable_labels.R
      set_derived_variable_labels.RdTakes labels from the Derived Variables CSV file and applies them to the
passed data frame.
The excel sheet must have columns "df_name", "df_label", "var_name", and "var_label".
The returned data frame is also labelled with the value in "df_label".
Examples
if (FALSE) {
trial %>%
  set_derived_variable_labels("derived_variables_sjoberg.csv")
}