function finish(err, root) {
        /* istanbul ignore if */
        if (!callback)
            return;
        var cb = callback;
        callback = null;
        if (sync)
            throw err;
        cb(err, root);
    }
Loads one or multiple .proto or preprocessed .json files into this root namespace and calls the callback.
@param {string|string[]} filename Names of one or multiple files to load
@param {IParseOptions} options Parse options
@param {LoadCallback} callback Callback function
@returns {undefined}