Increments level difficulty (length and nesting) by 1 until maximum.

def update(self, loss, force=False):
    """Increments level difficulty (length and nesting) by 1 until maximum."""
    do_update = super(CombineCurriculum, self).update(loss, force)
    if not do_update:
      return False

    if self._curr_length < self._max_length:
      self._curr_length += 1
    elif self._curr_nesting < self._max_nesting:
      self._curr_nesting += 1
    else:
      self._set_new_task = False

    if self._set_new_task:
      tf.logging.info("New level: (length=%d, nesting=%d)",
                      self._curr_length, self._curr_nesting)
    return self._set_new_task