GRIB2 load for GDT90 (space view).

# Warning! Contains trigonometry! ☣

The following images might help when considering the determination of the projected, visible disk dimensions:

![geostationary_top](https://cloud.githubusercontent.com/assets/917914/5218943/29193f6e-764a-11e4-809a-ab3a15e42ba4.png)

![geostationary_side](https://cloud.githubusercontent.com/assets/917914/5218944/2ce8292a-764a-11e4-83a7-f2c9d8975455.png)

any altitude less than 10s of km is going to be a pretty poor view, but I guess 0 causes numerical issues, so we need to trap that one.

I think we should consider the case where Nr is set to missing, as this is explicit in the Spec as being an orthographic projection, which we could handle, but with more code.
We could trap it and raise an exception or implement a new code path (or implement the extra path later, if it's ever needed)