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.
I don't think we should be raising an error on this. For example, a message might have all three of radius, major, and minor defined, but also shape==0. As far as the GRIB spec. is concerned that still equates to a sphere of radius 6367470 m.