Add an LFN table to a parsed LIGO_LW XML document.

    lfn = lfn to be added

def add_lfn(self,lfn):
    """
    Add an LFN table to a parsed LIGO_LW XML document.

    lfn = lfn to be added
    """
    if len(self.table['process']['stream']) > 1:
      msg = "cannot add lfn to table with more than one process"
      raise LIGOLwParseError, msg
    # get the process_id from the process table
    pid_col = self.table['process']['orderedcol'].index('process_id')
    pid = self.table['process']['stream'][0][pid_col]
    try:
      self.table['lfn']['stream'].append((pid,lfn))
    except KeyError:
      self.table['lfn'] = {
        'pos' : 0,
        'column' : {'process_id' : 'ilwd:char', 'name' : 'lstring'},
        'stream' : [(pid, lfn)],
        'query' : '',
        'orderedcol' : ['process_id', 'name' ]
        }