(re-)group all logevents by the given group.

def group(self):
        """(re-)group all logevents by the given group."""
        if hasattr(self, 'group_by'):
            group_by = self.group_by
        else:
            group_by = self.default_group_by
            if self.args['group'] is not None:
                group_by = self.args['group']

        self.groups = Grouping(self.logevents, group_by)
        self.groups.move_items(None, 'others')
        self.groups.sort_by_size(group_limit=self.args['group_limit'],
                                 discard_others=self.args['no_others'])