Allow to specify ecCodes key type

[sandorkertesz]

Some ecCodes keys like centre can be accessed both as number and string. It would be great if pdbufr could support the ecCodes type notations (see e.g. bufr_ls) to specify the return type of the key values. The notation is simply based on adding the :[d,i,s] suffix to a key to specify a float, int or string return type. For example, for an ECMWF BUFR message centre:s would result in "ecmf" while centre:i would result in 98.