Find child, child's child and so on for class(es) using recursive programming
tree_descendants(classes, class, leafs)
Character vector with the descendants of a class.
The class definitions of a cellpypes object, i.e. obj$classes.
A character vector with one or multiple classes.
Has to be the output of tree_leaf_nodes(classes). It's passed as argument so that tree_leaf_nodes is not executed in each recursion.