Utilizes the EPOCH method from the SDTM spec: Missing when --DTC is blank;
equal to 'SCREENING' if --DTC if before RFXSTDTC; equal to 'TREATMENT'
if --DTC is on or after RFXSTDTC and on or before RFXENDTC; equal to
'FOLLOW-UP' if --DTC is after RFXENDTC.
a data frame with date class columns RFXSTDTC and RFXENDTC and
the column given in the date_col argument
date_col
a string, the column name of the event date used to determine
the EPOCH; this column can either have a date class or a character class in
the YYYY-MM-DD format
RFXSTDTC
a string, the date column to use for RFXSTDTC, default is
"RFXSTDTC"; this column can either have a date class or a character class in
the YYYY-MM-DD format
RFXENDTC
a string, the date column to use for RFXENDTC, default is
"RFXENDTC"; this column can either have a date class or a character class in
the YYYY-MM-DD format