Speed-up interaction calculations

[mayer79]

The interaction calculations are slow. Here is my plan to make it faster:

• [x] Unify output of getTree() and treeInfo() and adapt code correspondingly. Most functions that distinguish {randomForest} and {ranger} can be removed. Implemented by #41
• [ ] Simplify code basis, e.g., reduce the amount of {dplyr} code.
• [ ] Make interaction calculations faster.

@pbiecek @hbaniecki