various updates of 'simultaneous_fit'

- new function '.filter_groups' filters groups based on their red. chi-sqr
- new function '.select_groups' sets default groups to be worked on
- new function '.group_stats' prints a histogram of red. chi-sqr values
- print such a histogram after .eval_groups and .fit_groups
- changed the convergence criterium of .fit_smart to chi-sqr
- .fit_smart is slightly faster now
- .plot_group without any argument plots the currently selected group
- don't be verbose during .eval_groups by default anymore