allow to set XDR mappings from user tasks

lib/api/GwApi.h

@@ -5,6 +5,7 @@
 #include "NMEA0183Msg.h"
 #include "GWConfig.h"
 #include "GwBoatData.h"
+#include "GwXDRMappings.h"
 #include <map>
 //API to be used for additional tasks
 class GwApi{
@@ -151,6 +152,11 @@ class GwApi{
         virtual void remove(int idx){}
         virtual TaskInterfaces * taskInterfaces()=0;
 
+        /**
+         * only allowed during init methods
+        */
+        virtual bool addXdrMapping(const GwXDRMappingDef &);
+
         /**
          * not thread safe methods
          * accessing boat data must only be executed from within the main thread