tree_apply: Apply a function to each node of a tree
In paulponcet/oak: Trees Creation and Manipulation

tree_apply applies a function fun to each node of .tree and stores the results in the attribute at.

tree_apply(.tree, ...)

## S3 method for class 'rtree'
tree_apply(.tree, at, fun, ...)

`.tree`	A tree.
`...`	Additional arguments to be passed to `fun`.
`at`	character. Name of the attribute to be created at each node of `.tree` that will contain the result of `fun`.
`fun`	function or character. A function taking two arguments `.node` and `.tree` (in this order), to be applied to each node of the tree.

## Rooted tree
(tr0 = c_("Bob", "Carl", "Daniel"))
(tr1 = c_("Bill", "Caroline", "Dimitri", "Enoc"))
(tr2 = r_("Alice", s = list(tr0, tr1)))

## Unrooted tree
(tr3 = r_(s = list(tr2, c_("Grand-Mother", "Father", "Son"))))

f <- function(.node, .tree) nchar(label(.node))
tr4 = tree_apply(tr3, at = "value", fun = f)
print(tr4, at = "value")

g <- function(.node, .tree) height(take_branch(.tree, .node))
tr5 = tree_apply(tr3, at = "height", fun = g)
print(tr5, at = "height")