Add node points to a clustering tree plot with the specified aesthetics.
add_node_points(node_colour, node_size, node_alpha, allowed)
either a value indicating a colour to use for all nodes or the name of a metadata column to colour nodes by
either a numeric value giving the size of all nodes or the name of a metadata column to use for node sizes
either a numeric value giving the alpha of all nodes or the name of a metadata column to use for node transparency
vector of allowed node attributes to use as aesthetics