Add shape of earth 1 and 7

This PR extends `ellispsoid` to handle `shapeOfTheEarth` value `1` and `7`.

This is a precursor to GDT 4/5.
Given what we found in https://github.com/SciTools/iris/pull/1364, I think we may have a problem coming here as to test for the right MDI value we need to know whether the key value is intended to be signed or unsigned.
This probably needs to be resolved in re-base after https://github.com/SciTools/iris/pull/1364 goes in ?

I think this routine can quite reasonably just check everything for both signed-  and unsigned-MDI-value.