Problem:
1. `ForwardNode::getConstNeighbor` returns a reference to a non-`const`
object, but the referenced pointer is `const`
2. `FowardNode::toNeighborRange` checks if the type of the argument is
const, but since `Successors` is not declared const the resulting
`toNeighborRange(Successors)` is never executed in the const version
Solution:
1. Make `getConstNeighbor()` return a reference to a `const` pointer
2. Explicitly declare a separate `toNeighborRange()` function which
returns a `const_child_iterator` and remove the template parameter
Alain Carlucci