Improve setting GRIB2 key `shapeOfTheEarth`

Improve grib saving of shape of the earth key

Improves the setting and testing of the GRIB2 Grid Definition Section key `shapeOfTheEarth`. 

Translation of three more key values has been added. These are (see GRIB Spec Code Table 3.2):
 * **0**: "Earth assumed spherical with radius = 6 367 470.0 m".
 * **6**: "Earth assumed spherical with radius of 6 371 229.0 m".
 * **9**: "Earth represented by the Ordnance Survey Great Britain 1936 Datum, using the Airy 1830 Spheroid,
the Greenwich meridian as 0 longitude, and the Newlyn datum as mean sea level, 0 height".

Some test results have been changed in line with these changes, where these changes have corrected the value that `shapeOfTheEarth` was previously and incorrectly being set to.

The shape of the earth tests in the tests for GDT 0, 1, 5 and 12 have also been refactored into their own test module to prevent duplicating the tests.