GRIB2 GDT 4.

This PR extends the new GRIB2 capability to include GDT 4.

- [ ] Rebase against #1354 
- [ ] Deal with MDI
Query the logic here matches to the grib spec and the grib-api definition: the note in the grib spec for setting this says:
> For ordinary cases, zero and missing values should be coded, equivalent to the respective values of 1 and 10^6
(10^–6 degrees unit)