helper functions for atctrees
helpers.Rdtreelevs: return levels within tree (excluding 0)
leaves: return labels of terminal elements (leaves) of tree
nlevs: number of levels in tree (excluding 0)
helpers.Rdtreelevs: return levels within tree (excluding 0)
leaves: return labels of terminal elements (leaves) of tree
nlevs: number of levels in tree (excluding 0)