tp_phylo_to_tpjson takes an object of class phylo and returns
a TreePPL json str ready to print in a data file for TreePPL use.
Arguments
- phylo_tree
an object of class ape::phylo.
- age
a string that determines the way the age of the nodes are calculated (default "branch-length").
"branch-length" : Root's age = NA, branch-length from the parent branch
"root-to-tip" : Root's age = 0.0, cumulative branch-length from root
"tip-to-root" : Leaf's age = 0.0, cumulative branch-length from leaf