This build a dict for fast retrive tasks id based on organisational tags. The form of the dict is:
        
            { 'org_tag_1': ['task_id', 'task_id', ...], 
              'org_tag_2' : ['task_id', 'task_id', ...], 
              ... }

def get_organisational_tags_to_task(self):
        """ This build a dict for fast retrive tasks id based on organisational tags. The form of the dict is:
        
            { 'org_tag_1': ['task_id', 'task_id', ...], 
              'org_tag_2' : ['task_id', 'task_id', ...], 
              ... }
         """
        if self._organisational_tags_to_task != {}:
            return self._organisational_tags_to_task

        for taskid, task in self.get_tasks().items():
            for tag in task.get_tags()[2]:
                self._organisational_tags_to_task.setdefault(tag.get_name(), []).append(taskid)

        return self._organisational_tags_to_task