public void addIndexField(String indexName, String fieldName) {
            FieldSet fs = fieldSets.get(indexName);
            if (fs == null) {
                fs = new FieldSet(indexName);
                fieldSets.put(indexName, fs);
            }
            fs.addFieldName(fieldName);
        }
Converts to the right index type from a field datatype
config-model/src/main/java/com/yahoo/schema/derived/VsmFields.java