[lib] XML models: add preamble with encoding during the export

library/cadbiom/models/guard_transitions/translators/chart_xml.py

@@ -240,9 +240,10 @@ class XmlVisitor:
             const.text = tnode.model.constraints
             xmodel.append(const)
 
-        self.xml = etree.tostring(xmodel, pretty_print=True)
-
-    #        print (etree.tostring(xmodel,pretty_print=True))
+        # Add preamble
+        self.xml = '<?xml version = "1.0" encoding="ASCII" standalone="yes" ?>\n'
+        self.xml += etree.tostring(xmodel, pretty_print=True)
+        # print(etree.tostring(xmodel,pretty_print=True))
 
     def visit_ctransition(self, trans):
         """