add default slotframe number of cells

[Xavier Vilajosana]

13. Even if the number of timeslots in a slotframe is variable for this minimal configuration, should there be a default?

TW> The danger with a default value is that it doesn't satisfy anyone. TW> We could indicate a default value (11? such as in the plugtest?) and indicate TW> that this value should be chosen to match the number of frames exchanged by TW> nodes per unit time. TW> @Editor, please add a default value and the note above.