debug console message added if callback function in ajaxCall() fkt. is invalid
* replace the text input field with an select element which contains the stations if we got more than 1 in getHaltestellen() method
* when we got the data from a station, we replace the select element with the initial text field, its value is set to the queried station string
added an id (input) to the div inside the form element which contains the input field and its label