Allow slots to return Slot_Duration equal to 0

[ssaezba]

Allow slots to return Slot_Duration equal to 0 to inform the scheduler that this slot has to be skipped without checking if the task is waiting or not. The behaviour has to be equivalent to an empty slot.

This will allow the user to effectively deactivate part of the plan. It can be useful to implement mixed criticality plans, where some slots are deactivated depending on the criticality level.

[ssaezba]

Adding an active status to the Work Control Block and Initial and Final Slots to activate/deactivate a Job as a whole