Speed up and simplify geography functions with geopandas

[leosaffin]

I've used the geopandas method from get_basin for the _get_natural_earth_feature function used by other functions in geography, which gives a substantial speed up.

I simplified the sjoin statement a bit by using predicate=contains, so you don't need the reset_index/groupby/first statements, which also gives a good speed up

get_basin now uses _get_natural_earth_feature because it is the same code now