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.

This new test passes, however there are a number of tests in the master branch
that fail.  I did not fix these, so running `mvn clean package` will fail.
It was failing before I made any changes.

Previously (and still) failing tests are:
```
Tests in error:
  testNullable(cascading.avro.AvroSchemeTest): unhandled exception
  testRoundTrip(cascading.avro.AvroSchemeTest): unhandled exception
  notUnpackedTest(cascading.avro.AvroSchemeTest): unhandled exception
  tupleInsideTupleTest(cascading.avro.AvroSchemeTest): unhandled exception
  testRoundTrip2(cascading.avro.AvroSchemeTest): unhandled exception
  groupByAvroReadWriteTest(cascading.avro.AvroSchemeTest): unhandled exception
  testSpecifiedColumns(cascading.avro.TrevniSchemeTest): unhandled exception
```