std::pair's to structures

I've replaced std::pair&lt;NodeID, NodeID&gt; and std::pair&lt;NodeID, bool&gt; with structs RestrictionSource and RestrictionTarget. I think now it's more readable and easier to understand.
Also replaced wrong type in CheckForEmanatingIsOnlyTurn method (in 'for' was RestrictionSource instead of RestrictionTarget).
This c'tor should be explicit.