Update the NCEP script to actually use the NCEP Reanalysis-2 data
(rather than the original reanalysis data).

The function now also downloads the geopotential height at the surface
used in the model. The original idea was that this would be able to mask
out the land data, but it turns out that the geopotential height does
not correlate with height above MSL very well, so it's difficult to mask
out the land areas. I've left the code in in case it's needed in the
future, for whatever reason.
Try to be clever about outputting the warning message when we are using the alternative grids
Move the topo to the start of the list of things to be downloaded so the longitude and latitude arrays are the right shape. Also select a non-topo field for the latitude and longitude arrays at the end
Add support for the new pressure field name