code: def gene_tracking_to_fpkm(tracking_file, out_file):
      
      if file_exists(out_file):
          return out_file
      df = pd.io.parsers.read_table(tracking_file, sep="\t", header=0)
      df = df[['tracking_id', 'FPKM']]
      df = df.groupby(['tracking_id']).sum()
      df.to_csv(out_file, sep="\t", header=False, index_label=False)
      return out_file
docs: take a gene-level tracking file from Cufflinks and output a two column
      table with the first column as IDs and the second column as FPKM for the
      sample. combines FPKM from the same genes into one FPKM value to fix
      this bug: http://seqanswers.com/forums/showthread.php?t=5224&page=2