thus allowing one to do hybrid URL/Registry code like...
  if($url) {
    $dbc = Bio::EnsEMBL::Hive::URLFactory->fetch($url, 'compara')->dbc;
    $dbname = $dbc->dbname();
  }
  else { $dbc = Bio::EnsEMBL::Registry->get_DBAdaptor($dbname,'compara')->dbc; }
  $fa = Bio::EnsEMBL::Registry->get_adaptor($dbname,'compara','Family');