This function extracts for a particular pathway all genes and
the numbers they are represented with in the KEGG network from the
corresponding KGML pathway file.
# S4 method for character
gene_name_number(x)A data.frame listing the genes included in the pathway with
their names as well as numbers used in KEGG database.
A character hsa identifier of the pathway for which gene
infomation should be extracted as used in KEGG database ('hsa00100').
Stefanie Friedrichs, Patricia Burger