Fix new GRIB data interpretation

@rhattersley we also need to inspect the `scanningMode` key in message section 3, octet 72 (reference GRIB2 flag table 3.4) to determine whether the 3rd bit (0x4) is set i.e. j points are consecutive (changing fastest), thus the shape would be `(Ni, Nj)` instead.

I suggest that we merge #1305 and you can use the `iris.fileformats.grib._load_convert.scanning_mode` convenience function to make like simple ...