code: def get_quant_NAs(quantdata, quantheader):
      
      out = {}
      for qkey in quantheader:
          out[qkey] = quantdata.get(qkey, 'NA')
      return out
docs: Takes quantdata in a dict and header with quantkeys
      (eg iTRAQ isotopes). Returns dict of quant intensities
      with missing keys set to NA.