Hmm. Changing data types (and contents) of "computed keys", such as ```levelType```, has been problem for many years. However, I didn't expect "coded keys", such as ```indicatorOfTypeOfLevel```, to suffer from this as they should reflect the grib  spec, which states the data type. Because we've been using a computed key here for some time I can't be sure if this is a recent change or if it has always been decoded as a string (https://software.ecmwf.int/issues/browse/SUP-484).

I suggest we remove this vulnerability. We can force ```GribWrapper``` to give us the coded **integer** value for this key in ```GribWrapper.__getattr__```, as we currently do for ```typeOfFirstFixedSurface```.