fix(dcc document): add namespace when dcc is created without xml

src/DCCDocument.ts

 import { DigitalCalibrationCertificate } from "./DigitalCalibrationCertificate";
 import { js2xml, xml2js } from "xml-js";
+import { XMLElement } from "./XMLElement";
 
-export class DCCDocument {
+export class DCCDocument extends XMLElement {
   _declaration: any;
   digitalCalibrationCertificate: DigitalCalibrationCertificate;
 
   constructor(xml?: string) {
+    super();
+
     if (xml) {
       let parsed: any = xml2js(xml, {
         compact: true,
@@ -24,6 +27,12 @@ export class DCCDocument {
       this.digitalCalibrationCertificate = new DigitalCalibrationCertificate(parsed.digitalCalibrationCertificate);
     } else {
       this._declaration = { _attr: { version: "1.0", encoding: "UTF-8" } };
+      this._attr = {
+        "xmlns:xsi": "http://www.w3.org/2001/XMLSchema-instance",
+        "xsi:schemaLocation": "https://intranet.ptb.de/fileadmin/dokumente/intranet/abteilungen/abteilung_1/Digitaler_Kalibrierschein/DCC dcc.xsd",
+        "xmlns:dcc": "https://intranet.ptb.de/fileadmin/dokumente/intranet/abteilungen/abteilung_1/Digitaler_Kalibrierschein/DCC",
+        "xmlns:si": "https://intranet.ptb.de/fileadmin/dokumente/intranet/abteilungen/abteilung_1/Digitaler_Kalibrierschein/SI",
+      };
       this.digitalCalibrationCertificate = new DigitalCalibrationCertificate();
     }
   }