docs: def write(self, outfile='initial_abundance.dat',
                header_string='initial abundances for a PPN run'):
          '''
          Write initial abundance file (intended for use with ppn)
  
          Parameters
          ----------
          outfile : string
              Name of output file.  The default is
              'initial_abundance.dat'.
          header_string : string
              A string with header line.  The default is
              'initial abundances for a PPN run'.
  
          '''
  
          dcols=['Z', 'species','mass fraction']
          data=[self.z,self.names,self.abu]
          hd=[header_string]
          att.write(outfile,hd,dcols,data)