Dominance checking for expansion candidates

[dharabor]

Nodes that are popped from the queue might be dominated by other nodes from the OPEN list. We need a procedure to check for dominance and avoid expanding any nodes which are dominated. We can interpret the dominance checking procedure as the application of a constraint that performs a feasibility cut.

Proposal:

1. Unidirectional search receives a new template parameter called CONSTRAINTS.
2. Objects which are instantiated as constraints must implement a prune function.
3. By default, we have NO_CONSTRAINTS instantiation.