Speed-up interaction calculations

[mayer79]

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

• Unify output of getTree() and treeInfo() and adapt code correspondingly. Most functions that distinguish {randomForest} and {ranger} can be removed. Implemented by Unify forests 1 #41
• Simplify code basis, e.g., reduce the amount of {dplyr} code.
• Make interaction calculations faster.

@pbiecek @hbaniecki