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
Reference Code Table 3.2
We could ditch the `radius` and assume that a `major` with no `minor` is a radius.

> These parameters aren't passed in either of the calls to this function

I'm paving the way for GDT 4/5 and 90. Seems disconnected, but I'm trying to push little and often rather than big with a slow turnaround.