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)

@rhattersley Consider if we can easily document/help the reader here ... decompose?