Test, update, and document code for interacting with message-ix-buildings

[khaeru]

.model.buildings was migrated from message_data in #350. This code was developed to work with the contents of the older iiasa/MESSAGE_Buildings repository (not public).

In the interim, the public iiasa/message-ix-buildings repository has superseded MESSAGE_Buildings. This has some improvements—for example, a pyproject.toml so that the repo contents can be installed from source—but also other changes that, like the repo as a whole, are not documented. Prima facie, we do not expect the migrated .model.buildings code to work with the new repository.

Some next steps may include:

• Add tests that run individual pieces in .model.buildings.
• Identify the pieces that do and do not currently work with the new repo; apply test marks.
• Update the code to work with the current interface of message-ix-buildings.
• In the absence of documentation for message-ix-buildings, document how message_ix_models.model.buildings expects that other code to behave.