* Allows for clearer errors when a GeoDataFrame is created but has no
geometry.
* TST: Add test util for comparing sequence (b/c assert_almost_equal seems
to not work with shapely.
Introduce type testing for GeoDataFrame.
Add tests for boolean index.
GeoDataFrame.__getitem__(key) returns the Geo* equivalent class when a
'geometry' column is present.
The following are True when df is a GeoDataFrame:
type(df['geometry']) is GeoSeries
type(df['column']) is Series
type(df[['column1', 'column2', 'geometry']]) is GeoDataFrame
type(df[['column1', 'column2']]) is DataFrame