This function takes an array of SYMBOL and convert it into GENE ENSEMBL.
get_Symbol2GeneEnsembl(symbol.arr, dataset = "mmusculus_gene_ensembl")
a character array. The SYMBOL array.
must be either "mmusculus_gene_ensembl" or "hsapiens_gene_ensembl".
A data frame contains 3 columns, "Gene", original ID, "Symbol" and "description".