change & deprecate .aggregate api to mimic pymongo's interface + separate the aggregation tests from the large test_queryset.py file

docs/guide/querying.rst

@@ -400,7 +400,7 @@ would be generating "tag-clouds"::
 
 MongoDB aggregation API
 -----------------------
-If you need to run aggregation pipelines, MongoEngine provides an entry point `Pymongo's aggregation framework <https://api.mongodb.com/python/current/examples/aggregation.html#aggregation-framework>`_
+If you need to run aggregation pipelines, MongoEngine provides an entry point to `Pymongo's aggregation framework <https://api.mongodb.com/python/current/examples/aggregation.html#aggregation-framework>`_
 through :meth:`~mongoengine.queryset.QuerySet.aggregate`. Check out Pymongo's documentation for the syntax and pipeline.
 An example of its use would be::
 
@@ -414,7 +414,7 @@ An example of its use would be::
             {"$sort" : {"name" : -1}},
             {"$project": {"_id": 0, "name": {"$toUpper": "$name"}}}
             ]
-        data = Person.objects().aggregate(*pipeline)
+        data = Person.objects().aggregate(pipeline)
         assert data == [{'name': 'BOB'}, {'name': 'JOHN'}]
 
 Query efficiency and performance