Support for NetCDF files on Geostationary Projections

[lbdreyer]

The Geostationary grid mapping was added in CF conventions 1.7.

This has been raised by a number of our users in the satellite community. Increasingly satellite products on this type of projection are becoming available in the satellite meteorology community so it would be very helpful to support the Geostationary grid_mapping correctly.

For more background on the discussions adding the projection to CF 1.7, you can view the original discussion of its inclusion here.

(Let me know if you need help getting hold of example netCDF files on a geostationary projection!)

[lbdreyer]

Currently Iris has iris.coord_system.VerticalPerspective coord system which points off to the ccrs.Geostationary, which is a geos proj4 projection.

The cf conventions suggest that the Vertical Perspective projection corresponds to the nsper proj4

A general description of vertical perspective projection is given in [Snyder], pages 169-181.
The corresponding projection in PROJ.4 is nsper. This should not be confused with the PROJ.4 geos projection.

(see Vertical Perspective)

I think Iris needs to be updated so that the iris.coord_system.VerticalPerspective uses ccrs.NearsidePerspective (which is nsper) and we add a iris.coord_system.Geostationary that uses ccrs.Geostationary (which is geos)

[lbdreyer]

Closed by #3406