Trees summary statistics: height, splits

[jxu]

There should be some functionality in ranger objects to show some tree summary statistics like tree height, number of splits, etc.
treeInfo shows all the splits, but no easy way to get summary info.

[mnwright]

Yes, that would be nice. What else do we need? Could add this to print() or create something like summary() for ranger objects.

[jxu]

I like #699