cf_writer fix let threaded_algorithm process command line

was forwarding to teca_algorithm which resulted in none of the threading
related properties being picked up from the command line.

io/teca_cf_writer.cxx

@@ -147,7 +147,7 @@ void teca_cf_writer::get_properties_description(
             "the list of non-geometric arrays to write")
         ;
 
-    this->teca_algorithm::get_properties_description(prefix, opts);
+    this->teca_threaded_algorithm::get_properties_description(prefix, opts);
 
     global_opts.add(opts);
 }