Create pass through model

[athowes]

It's a good test of our infrastructure that we can pass through to brms with as little additional model code as possible.

Doing this will also allow users to test our models in comparison to a baseline with no adjustments for biases in the data.

Some things I can spot we need to do to achieve this:

• epidist_prior passes through cleanly
• epidist_stancode passes through cleanly
• epidist_validate works for pass through model
• Some kind of shared preprocessing for pass through model

[athowes]

Flagging this as something to work on post #382.