createFetcher (url, fetchInterval, excludedEvents, maximumEntries, maximumNumberOfDays, auth, broadcastPastEvents, selfSignedCert, identifier) {
		try {
			new URL(url);
		} catch (error) {
			Log.error("Calendar Error. Malformed calendar url: ", url, error);
			this.sendSocketNotification("CALENDAR_ERROR", { error_type: "MODULE_ERROR_MALFORMED_URL" });
			return;
		}

		let fetcher;
		let fetchIntervalCorrected;
		if (typeof this.fetchers[identifier + url] === "undefined") {
			if (fetchInterval < 60000) {
				Log.warn(`fetchInterval for url ${url} must be >= 60000`);
				fetchIntervalCorrected = 60000;
			}
			Log.log(`Create new calendarfetcher for url: ${url} - Interval: ${fetchIntervalCorrected || fetchInterval}`);
			fetcher = new CalendarFetcher(url, fetchIntervalCorrected || fetchInterval, excludedEvents, maximumEntries, maximumNumberOfDays, auth, broadcastPastEvents, selfSignedCert);

			fetcher.onReceive((fetcher) => {
				this.broadcastEvents(fetcher, identifier);
			});

			fetcher.onError((fetcher, error) => {
				Log.error("Calendar Error. Could not fetch calendar: ", fetcher.url(), error);
				let error_type = NodeHelper.checkFetchError(error);
				this.sendSocketNotification("CALENDAR_ERROR", {
					id: identifier,
					error_type
				});
			});

			this.fetchers[identifier + url] = fetcher;
		} else {
			Log.log(`Use existing calendarfetcher for url: ${url}`);
			fetcher = this.fetchers[identifier + url];
			fetcher.broadcastEvents();
		}

		fetcher.startFetch();
	}
Creates a fetcher for a new url if it doesn't exist yet.
Otherwise it reuses the existing one.
@param {string} url The url of the calendar
@param {number} fetchInterval How often does the calendar needs to be fetched in ms
@param {string[]} excludedEvents An array of words / phrases from event titles that will be excluded from being shown.
@param {number} maximumEntries The maximum number of events fetched.
@param {number} maximumNumberOfDays The maximum number of days an event should be in the future.
@param {object} auth The object containing options for authentication against the calendar.
@param {boolean} broadcastPastEvents If true events from the past maximumNumberOfDays will be included in event broadcasts
@param {boolean} selfSignedCert If true, the server certificate is not verified against the list of supplied CAs.
@param {string} identifier ID of the module