Add __iter__ method to FeatureCollection and GeometryCollection
Add the __iter__ method to allow looping over the contents of a FeatureCollection or Geometry collection. Also added some examples to the docs:
>>> [geom for geom in geoms]
[Point(1.0, -1.0), Point(1.0, -1.0)]
>>> [feature for feature in c]
[<Feature Instance Point geometry 2 properties>, <Feature Instance Point geometry 2 properties>]
This allows you to loop over the Feature and Geometry objects contained in _features and _geoms without doing the following:
[geom for geom in geoms._geoms]
