MSP: Pareto set returns set of inserted elements
MSP: Spawn new tasks for non-dominated paths
MSP: Mark the graph and source node const
MSP: Add a class mapping pareto sets to graph nodes
MSP: Access pareto sets via SetMapper

Pareto sets are node local. Only paths to the same node
should be added to a pareto set.
MSP: Make Node::out_edges() thread-safe

Unfortunately, cgraph is not thread-safe. In particular, out_edges() was
causing segfaults through parallel use.
MSP: Abstract class needs virtual dtor
MSP: const after type
MSP: Init SetMapper with a graph pointer

It's more natural in this case, because graphs are passed as pointers
everywhere.
MSP: Mark SetMapper dtor virtual just to be safe
MSP: Adjust SetMapper include guard name