Extended the Mf.bug_21079_local_tm_global_selection system test for all Monte Carlo simulation steps.
Modified the create_mc_data() method to partly fix bug #21079 (https://gna.org/bugs/?21079).

Some spins with local tm models remain selected despite not containing any data.  These are handled
explicitly.  Instead of a RelaxNoModelError being raised, the method returns None to indicate that
something went wrong.
Final fix for bug #21079 (https://gna.org/bugs/?21079).

This is the failure of the dauvergne_protocol auto-analysis when the "local tm" global model is
selected.  The Monte Carlo create_data() method not skips data from the base_data_loop() if the
create_mc_data() method returns None.