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 the shapeOfTheEarth either one of three lookups are performed:
> 
>  -   default to a nailed down sphere/spheroid (no kwargs required)
>  -   the shape is an oblate spheroid with custom major/minor
>  -   the shape is a sphere with custom radius

But isn't it the job of this function to determine which of the three options should be applied depending on the value of the `shapeOfTheEarth` parameter? In which case the *calling* routine always has to pass all the values?

In other words, how would a GDT (e.g. GDT 0 which already exists) make use of this?