src/main/java/com/conveyal/gtfs/loader/Table.java
public List<Field> specFields () {
        List<Field> specFields = new ArrayList<>();
        for (Field f : fields) if (f.requirement == REQUIRED || f.requirement == OPTIONAL) specFields.add(f);
        return specFields;
    }
public List<Field> specFields () {
        List<Field> specFields = new ArrayList<>();
        for (Field f : fields) if (f.requirement == REQUIRED || f.requirement == OPTIONAL) specFields.add(f);
        return specFields;
    }
Get only those fields included in the official GTFS specification for this table, i.e., filter out fields used
in the editor or extensions.