Sum of geom in FeatureGrid

As the title says it.

Picture:
![sum_geom](https://cloud.githubusercontent.com/assets/1792111/3979110/937bf13c-2850-11e4-9df5-483342b4a251.png)

Don't you want to use that map.getUnits() info? This way the other systems will be supported out-of-the-box?