function loadTimetableData(dataUrl, clock) {
    return Promise.all([
        getTimetableFileName(clock),
        ...getExtraTimetableFileNames(clock)
    ].map(fileName => `${dataUrl}/${fileName}`).map(loadJSON)).then(data => [].concat(...data));
}
Load the timetable data.
@param {string} dataUrl - Data URL
@param {Clock} clock - Clock object representing the current time
@returns {Object} Loaded timetable data