Update for ETL will be needed to support modifer terms

[pnrobinson]

Currently, this code in EtlTools assumes that all non O/E/na entries are ages of onset

 for hpo_duplet in all_hpo_duplets {
            match hpo_to_status_map.get(hpo_duplet) {
                Some(status) => {
                    match status.as_str() {
                        "observed" => { values.push(CellValue::Observed);},
                        "excluded" => { values.push(CellValue::Excluded);},
                        "na" => { values.push(CellValue::Na);},
                        _ => { values.push(CellValue::OnsetAge(status.clone()));}
                    }
                }
                None => {
                    values.push(CellValue::Na);
                }
            }
         }