Add license headers to all files

[nuest]

Template below, or take a good look at https://reuse.software/tutorial/

/**
 * 
 * js/journal.js
 *
 * Copyright (c) 2022 OPTIMETA project
 * Copyright (c) 2022 Daniel Nüst
 * Distributed under the GNU GPL v3. For full terms see the file LICENSE.
 * 
 * @brief Display spatio-temporal metadata for a whole journal on a separate page.
*/