Limit a unit schedule to one elevation

Add a unit schedule and choose an elevation in its Filters so it lists only the modular items placed in that elevation.

1. When a unit schedule is added it lists units for the whole project by default.

2. Selecting an elevation in the schedule's Filters limits the schedule to the modular items placed in that elevation.

3. The schedule's Filters are shown as 'not set' until an elevation is chosen.