tree_age_cumul takes an tppl_tree and returns a json with
branch length accumulation as node's age.
Arguments
- tree
an tppl_tree create with tp_phylo_2_tppl_tree.
- root_index
the index of the root in the tppl_tree.
- age
a string that determine the way the age of the node are calculated (default "branch-length").
"top-down" : Root's age = 0.0, cumulative branch-length from root
"down-top" : Leaf's age = 0.0, cumulative branch-length from leaf