DOC: add basic documentation to some of the GroupBy properties and methods
DOC: add GroupBy class to api.rst
DOC: add import for GroupBy object to api.rst
DOC: add GroupBy.aggregate and GroupBy.transform to api.rst
DOC: add meta comment to api.rst about when GroupBy objects are created
DOC: add links to api.rst
DOC: remove aggregate/transform from api.rst