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.
Seems reasonable, but a couple of things to consider. firstly `np.ma.masked_equal` sets the fill value to the the provided value (this appears contrary to the example in the documentation) so you may want to consider what the dreaded fill value should be. Also, the MDI values in #1364 may be outside the range of int32/uint32 so the comparison might need to be done before casting to int32.