Allow user to supply a raster/polygon shapefile instead of table of point locations

It would be great that `climr` functions (e.g. `climr_downscale`) accepted a polygon shapefile or a raster layer instead of a table of point locations.

things to consider:
- would this be implemented in `climr_downscale` only or in inner functions (e.g. `downscale`)?
- if providing a polygon, `climr` will need to get/use its own DEM -- document where it comes from and its attributes
- if providing a raster, do we want to allow the user to:
1) provide a DEM?
2) a raster of cell locations AND a DEM?
3) or just a raster of locations and _always_ use `climr`'s default DEM?

note that in options 2 and 3, the attributes of the target raster (e.g. resolution, projection) could differ from the DEM's