feat: adding support for saving the status of a DataService class

By passing a the "filename" keyword to the DataService init function,
one can save the state of the instance in this file. Quitting the service
(Ctrl+C) will dump the state of the instance in the file with given
filename. When starting the instance, it will load these values again.

src/pyDataInterface/server/server.py

@@ -173,6 +173,9 @@ class Server:
     async def shutdown(self) -> None:
         logger.info("Shutting down")
 
+        logger.info(f"Saving data to {self._service._filename}.")
+        self._service.write_to_file()
+
         await self.__cancel_servers()
         await self.__cancel_tasks()