Add model's latest variable

Add a variable that can be used to access a specific model's latest interval. 

Ex: I have `db.user` model. To access latest date string I would do:
```
WHERE
    ds = @DB__USER__LATEST_DS
```
The key concept is that `latest` is local to each model and therefore if you want to access a table that has history of values on it and you always want "latest" then you can use this variable.