Level of dependency

[mechatroNick]

Having level dependency level would be really to have:

• numberOfLevel() to expose how many level of dependency there are (3 levels if a -> b -> c, or 2 levels if a , b -> c)
• showLevel(level) to expose all the node at that dependency level (showLevel(2) = b if a -> b -> c)

The inspiration for this request is to be able to update 1 level of dependency at a time. This is useful for representing nested calculations. This is one way to do it and not necessarily the best, please let me know if there are better ways.