Fix grib gdt5 load and add save+load roundtrip test.

I wanted to test against explicit values, because that shows clearly what the data actually is.  However, it also made sense to me to cross-check those values against the original.  It's complicated by some specific things being different between the two, so I tried to handle those separately.
I could ditch the tests on the original data if you prefer.

Another approach would be to modify one cube in specific ways so that it can be directly compared with the other.  Unfortunately, though, the coordinate values tests would still be a bit clunky because of the rounding errors -- I'd have to test them as at present, but then overwrite the values to make one cube match the other.