Relax Slab requirement for crawlspace

[shorowit]

Currently we require a Slab for a crawlspace, since E+ requires every thermal zone to have a floor. While a crawlspace can physically have a slab, it is usually just dirt. So we should allow HPXML files where the crawlspace has no Slab, and then create the dirt floor automatically for the E+ model. This will allow the HPXML to better reflect reality.

I guess we should do this for basements too.

[shorowit]

To do this, we can get the dirt floor's area from the crawlspace ceiling. Getting the exposed perimeter will be a bit more effort -- we'll need to calculate it from the crawlspace's foundation walls. (Will need a fallback if it's an entirely above-grade crawlspace in which there are only Walls, and no FoundationWalls, since we won't have wall lengths then.)