Implement/demo a restricted VAR model

See description of use case / idea [here](https://delphi-org.slack.com/archives/C028AUR6MV1/p1649339631327549). This particular case actually sounds like it might be AR plus an ARX model (or is it VAR plus VARX?), so this may be an instance of composing models (a generalization of iterative 1-ahead models; see connected issue [here](https://github.com/cmu-delphi/epipredict/issues/262)).