Consider last instance in filter and aggregator

This fixes an issue when running the VHT classifier with prequential evaluation, where
by ignoring the last instance property in filter/aggregator, the topology exited before
evaluating all instances.

Can be tested with:
`bin/samoa local target/SAMOA-Local-0.3.0-SNAPSHOT.jar "PrequentialEvaluation -d /tmp/dump.csv -i 1000000 -f 100000 -l (com.yahoo.labs.samoa.learners.classifiers.trees.VerticalHoeffdingTree -p 4) -s (com.yahoo.labs.samoa.moa.streams.generators.RandomTreeGenerator -c 2 -o 10 -u 10)"`
We need to drain because the condition in line 295 sometimes evaluates to false. Do we actually need to branch there?