SBT combine improvements

[luizirber]

From #143 (comment):

• The current solution (create a new root) works well with binary trees, but if d>2 we could instead check to see if there is a position available near the root before inserting. This is more visible if you try to combine trees in sequence.

• A more powerful solution would be to do a depth-first search in the original trees and replicate what is found in the new one.