Feat/reroot trivial node removal

[mmolari]

Re-rooting can sometimes introduce trivial nodes, i.e. nodes with exactly one child and one parent, corresponding to the old root of the tree.
I added a function to remove such nodes. It should

• check whether the old root is now a trivial node
• if so, delete it and connect the parent-child nodes with a new edge, whose length is the sum of the two edges.