Add operator/ to join treepaths

Summary

Based on the discussion in !109 (merged) this MR adds an operator/ to concatenate treepaths with other treepaths or indices.

Example

auto tp1 = treePath(0,1) / 2;     // == treepath(0,1,2)
auto tp2 = 2_tp / treePath(1,_0); // == treePath(_2,1,_0)