Add test for return value of project()

The `Pipeline.project()` method should return the instance so that calls
to the aggregation methods can be chained together. This new test checks
that `project()` returns the instance.
Clean up

The `Pipeline` class was originally going to be named `Aggregate` so I
named all of the test instances `a` (just like `Model` test instances
are usually named `m`). Because the class name actually starts with a
'P', I'm renaming `a` to `p`.