[CF-metadata] polar stereographic projections

[Jonathan Gregory]

Dear Magi

The proj pages have some remarks about polar stereographic projections so it's
a pity they're inaccessible!

> I am just looking at the specs of the polar stereographic projection and
> am wondering how I can select the South Pole since I can only select a
> scale and the direction of the polar map.

I am not familiar with how it is calculated, but I think you must be right.
Perhaps we should have latitude_of_projection_origin in this projection, as
well as the vertical longitude and the scale factor.

Cheers

Jonathan