[DownPropagator]: Fixed incorrect behaviour in DownPropagator

Fixed a rather significant error in DownPropagator, which would result in
incorrect indices being used to send fitnesses down the tree.

Signed-off-by: Sebastian Lenartowicz <slenarto@uoguelph.ca>
[Examples]: Added a version of 3-level HierRR that uses propagation

Added a modified version of the 3-level hierarchical royal road
GA, which propagates fitness all the way down the tree, from top
to bottom.

Signed-off-by: Sebastian Lenartowicz <slenarto@uoguelph.ca>