crossover idea for raadtools

[mdsumner]

we need to do some VRT stuff with oisst, with the point being that a WarpedVRT can go through the warper just one time, and then we can sub out the URL/dsn/filename - we don't have to warp the entire time series at once in order to get that efficiency

also wbg to have a whatarelief python package to do what this one is doing, should actually be pretty easy with rasterio