Fix up the `treeapply` function

[richfitz]

We need an accumulate function (possibly call it a fold) and an apply function, and need to do this over the different iterator orders and over children etc, and with the target being a node or being the node data. So that we can do

treeapply(tr, order="post", target="data", function(x) x$foo)

to get all the foo elements out of the tree.

The current implementation is very very basic. Needs to work on subtrees, and allow iteration over at one of three levels:

• subtree (like sub_ iterators)
• node (like the basic iterators)
• data (over just the data members when given an rtree)

Decide on how naming of output should go (names=TRUE)?

Write methods for dplyr's %.%? Or too silly?

[richfitz]

Something using Rcpp::Function seems worthwhile here. Get this written soon.