Returns all the leaf nodes from a particular organism-specific GO-DAG. If empty (or if "BP"), all the general BP
GO-DAG leaf nodes will be returned. The supported organism are "Homo sapiens / Human", "Rattus norvegicus / Rat",
"Mus musculus / Mouse", "Danio rerio / Zebrafish", "Caenorhabditis elegans / Worm" ,"Arabidopsis thaliana / Cress",
"Saccharomyces cerevisiae / Yeast", "Schizosaccharomyces pombe / Fission yeast",
"Drosophila melanogaster / Fruit fly", "Escherichia coli / E.coli".