a character scalar (default is "members"). If "order"
than nleaves is based on length of order.dendrogram.
If "members", than length is trusting what is written in the
dendrogram's root attr.
"members" is about 4 times faster than "order".
The number of leaves in the tree
The idea for the name is from functions like ncol, and nrow.
Also, it is worth noting that the nleaves.dendrogram is based on
order.dendrogram instead of labels.dendrogram since the first is
MUCH faster than the later.
The phylo method is based on turning the phylo to hclust and than to
dendrogram. It may not work for complex phylo trees.