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

Yes.
I get :
```
AssertionError: Failed on coordinate "grid_latitude" : 
Not equal to tolerance rtol=1e-07, atol=0

(mismatch 100.0%)
 x: array([ -5.59320000e+00,  -5.55720000e+00,  -5.52120000e+00,
        -5.48520000e+00,  -5.44920000e+00,  -5.41320000e+00,
        -5.37720000e+00,  -5.34120000e+00,  -5.30520000e+00,...
 y: array([ -5.59320021e+00,  -5.55719995e+00,  -5.52120018e+00,
        -5.48519993e+00,  -5.44920015e+00,  -5.41319990e+00,
        -5.37720013e+00,  -5.34119987e+00,  -5.30520010e+00,...
```

Of course, if we were to optimise the scaling this might improve :wink: