Support rank notation in ecCodes keys

[sandorkertesz]

ecCodes supports the #n#key notation to get the n-th value for a given key in a BUFR message. It would be great if pdbufr allowed for this notation. E.g this code

columns = ["latitude", "longitude", "#1#airTemperature"] 
res = pdbufr.read_bufr(path, columns=columns)

could return only the first airTemperature value from each message in a radiosonde BUFR file.