[CF-metadata] proposed UTM projection

[Jonathan Gregory]

Listening to John and Rich, it seems to me that we should

* Extend the transverse Mercator to allow a UTM zone, say how this specifies
TM parameters, and that the two ways are mutually exclusive (specifying both
would be a CF error).

* Add a definition of spheroid to any projection definition where it is
relevant, with a list of permitted values. In John's original proposal, he
gave numbers to define the ellipsoid (semimajor axis and inverse flattening).
Rich has given names instead. Are these alternatives? Is there a definitive
source we could refer to?

Jonathan