Added PackedAvroSchemeCascadingTest to demo how to use PackedAvroScheme

This required having the maven `generate-test-sources` target write to
`src/test/java` rather than to `target`, so all of the avro Objects
generated are now there and added to the repo.

Created a new src/test/resources/cascading/data directory for the inputs
to the two tests in PackedAvroSchemeCascadingTest.

PackedAvroSchemeCascadingTest is passing.