Version tracking with OpenStudio-BuildStock

[nmerket]

It's hard to keep track of what version(s) of the OpenStudio-BuildStock this works with. There should be some way we can pin that while still allowing some flexibility for people to hook it up to different versions. Additionally it makes the setup difficult when you have to check out two repositories. This may be a down the road kind of issue as these projects merge, so it'd be better to wait until then.

Ideas:

• git submodule 😱
• git subtree 😨
• including a commit hash for OpenStudio-BuildStock in this somewhere. 🤢

[nmerket]

Mostly addressed with #318.