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)

> I think we should consider the case where Nr is set to missing

Definitely. I was taking that as read as it was my original intention.