* Added a properties for raster representation (via the rasterize method)
and vector outline (via shapely's' cascaded_union).
* Re-factor the package into utilities.py and gridgeo.py.
* Removed precision argument. (Created degenerated polygons.)
* The __repr__ now return the original grid __repr__ if u/s-grid
or the original nc object if the grid is "unknown."
* Simplified the GridGeo class.
(Moved all the loading stuff to load_grid.)
* Caching expensive computations.