Returns the current status of an asynchroneous routing process. It can be used for polling the progress of a routes calculation. An initial process needs to be started beforehand via POST calcroute/optimized/timewindow or (for Excel uploads) POST /upload/{routingtype}. The returned process ID is used for identifying this process. The attribute {state} indicates if an optimization is still running ('PENDING'), successfully finished ('SUCCESS') or failed ('FAILURE').