read_codelist() reads a code list – one row a value of a variable:
variable, value, label (what it prints as), order (its place)
and under (a nested count's inner value: the value, as the data have
it, of the variable one level out it sits under, for
complete_hierarchy()) – from an .xlsx (its first sheet) or a .csv
file. set_codelist() puts it into a report's rows of the definition's
codelists sheet: a value the report has already for the same variable
is replaced, the others are kept (rows with no under column keep the
under the report's values have). A code list is a report's (tflspec:
every row names its report); to use one in several reports, copy it
(import_codelist(), standard_codelists()).