Add GRIB2 workaround helper for int32->uint32.

Partner in crime to #1480.
I've plumped for, "no".

GRIB2 is always big-endian. We can assume the ECMWF GRIB API takes care of this when operating on a little-endian platform - i.e. the Python `int` returned by `grib_get` should have the same logical value on all platforms. We can also assume `grib_set` takes care of byte-order when mapping from the logical value of a Python `int` to the on-disk bytes. Since the transformation in this fixup function is operating on the logical values (not any particular bit pattern) it is independent of the native byte order. QED. [If there were a crossed-fingers emoticon I'd used it here!]