cett_nodes_options      returns carriers, enzymes,targets and
                        transporters node valid options.
drug_node_options       returns drug node valid options.
parseDrugBank           parseDrugBank
references_node_options
                        returns references node valid options.
show_dvobject_metadata
                        init_dvobject Returns data.frame with two
                        columns (key, value) of dvobject attributes
