Studio.is_scheduling is now checked by the user

0.2.6.9
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* **Update:** ``Studio.is_scheduling`` and ``Studio.is_scheduling_by``
  attributes will not be updated or checked at the beginning of the
  ``Studio.schedule()`` method. It is the duty of the user to check those
  attributes before calling ``Studio.schedule()``. This is done in this way
  because without being able to do a db commit inside ``Studio.schedule()``
  method (which is the case with transaction managers which may be used in web
  applications like **Stalker Pyramid**) it is not possible to persist and thus
  use those variables. So, to be able to use those attributes meaningfully the
  user should set them. Those variables will be set to False and None
  accordingly by the ``Studio.schedule()`` method after the scheduling is done.