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.
@pp-mo I think that you've got an old version of the GRIB spec ...

```
Code table 3.2 – Shape of the Earth
...
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
```

`shapeOfTheEarth = 9` is a valid value but we don't support it (as yet)

Fair comment about MDI, which is starting to infiltrate a lot of code now as we move forward. I'm going to compromise here. I like the distinction between `unsupported` and `invalid` so I'm going to opt for anything `invalid` as outside the range of `0...9`