The function retrieves the Regular nodes (RNs) from a GO-level. RNs are those GO-terms which have all their
children in the next GO-level (i.e level = level of parent + 1). If the optional
parameter "organism" is missing
then the result will be from the general GO tree instead of from a particular organism. The supported organisms
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".