Initializes object with its default values

        Tries to load self.default_filename from default
        data directory. For safety, filename is reset to None so that it doesn't point to the
        original file.

def init_default(self):
        """
        Initializes object with its default values

        Tries to load self.default_filename from default
        data directory. For safety, filename is reset to None so that it doesn't point to the
        original file.
        """
        import f311
        if self.default_filename is None:
            raise RuntimeError("Class '{}' has no default filename".format(self.__class__.__name__))
        fullpath = f311.get_default_data_path(self.default_filename, class_=self.__class__)
        self.load(fullpath)
        self.filename = None