Add breadth first traversal to mirror the depth-first traversal in SelectRecursive

[kevmoo]

Implementation thoughts:

1. Add a SelectRecursiveAtLevel method that only returns items at a provided depth n
2. For the breadth first version, just call SelectrecursizeAtLevel for 0 -> N (where we stop once an empty level is found)
3. Allows for a breadth first search where one can provide a max level paramater
4. To keep the implementation cheap, this would require multiple iterations of the provided source collection
   • This is probably better than the potentially huge state the method would have to maintain to keep track of where the traversal is