Allow describing mass floors

[shorowit]

Spun off from the discussion in #298.

For multifamily buildings, an apartment floor can be a mass floor instead of a frame floor. Some options for handling this in HPXML:

1. Create a new MassFloor element, at the same level as the FrameFloor element, with inputs specific to mass floors.
2. Rename the FrameFloor element to Floor and introduce a FloorType child element (similar to Wall/WallType).
3. Use the Slab element to describe a mass floor? At a minimum, it would need an ExteriorAdjacentTo element.

[nmerket]

After much deliberation, we think 2 is the best option.

[nmerket]

🎥 Discussion in HPXML Working Group meeting 2022-06-24

[nmerket]

🎥 Discussion in HPXML Working Group Meeting 2022-07-12

[nmerket]

🎥 Discussion in HPXML Working Group Meeting 2022-10-11