The function retrieves the Regular nodes (RNs) from a GO-level. RNs are those GO-terms which have all their
children term in the next GO-level (i.e 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".