BF - Correct Earth radius when saving GRIB2 files, fixes #690.

This PR fixes a problem where the Earth's radius in the coordinate system was accidentally set to 0 when saving GRIB2 files. This turned out to be due to an issue when loading files where the scale factor was interpreted incorrectly.

To-do:

- [x] update test data reference when SciTools/iris-test-data#20 is merged
- [x] update change log and what's new when accepted in principle