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.
> missing values always come back as either -2 "billion" or 4 "billion".

i.e. experiments with the ECMWF GRIB API show that negative numbers *are* written as sign-and-magnitude, *not* two's complement.