fdsn: fix default parameters
fdsn: change get_event into get_events

 - returns a catalog so get_event is misleading
 - same for get_station -> get_stations
fdsn: fix handling of parameter short aliases

also some refactoring, remove code duplication
fdsn: start adding tests for example queries
fdsn: except ignored parameters, show warning
fdsn: finish example query tests for "event", "station"