Add Walkout basement element

[shorowit]

Adds a Walkout boolean element for the basement foundation type.

[nmerket]

Wouldn't any garage that's a foundation be a "walk out"? If you can drive out of it, you can walk out of it. We could use some more clarification of when this would be used and if it's needed. We could also use some clarification and documentation on walkout basement.

[nmerket]

:movie_camera: Discussion in HPXML Working Group Meeting 2023-06-13

[shorowit]

• [x] Update description to refer to a door directly on grade (so as to exclude doors that have stairs to the ground).

[nmerket]

:movie_camera: Discussion in HPXML Working Group Meeting 2023-07-11