Smarter way to build "dv_historic.rds"

[lindsayplatt]

Right now, changing the dates will force the historic date to rebuild, which is not always what we want. It currently depends on sites and dates. So if either of those change, it will rebuild. We should have a single dv_historic process that is not dependent on many other changing pieces so that people can rebuild this with a different time period or extent more quickly.