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.
Discussed with @bjlittle offline. Summary of findings:
 - `gribapi` always returns 64-bit integers (irrespective of signed/unsigned and bit-width in the message).
 - So missing values *always* come back as either -2 "billion" or 4 "billion".

So....
 - Get rid of checks like np.int8(value) == _CODE_TABLE_MDI.
 - Raise TranslationError if missing values in array.
 - Fix everything in a separate PR :wink: and then rebase this PR.