Extents
- Compute extent from feature collection
- Compute extent from table
- Compute extent from vector of geometries
All of this is made a lot easier with GO.apply which might become GI.apply or GI.map at some point...
Boundaries
Shapely has a boundary method that extracts exterior rings as linestrings.
We do also need to create constructors such that GI.LineString can wrap LinearRing trait objects.