code: def get_splitcolnr(header, bioset, splitcol):
      
      if bioset:
          return header.index(mzidtsvdata.HEADER_SETNAME)
      elif splitcol is not None:
          return splitcol - 1
      else:
          raise RuntimeError('Must specify either --bioset or --splitcol')
docs: Returns column nr on which to split PSM table. Chooses from flags
      given via bioset and splitcol