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

[CeresBarros]

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

[kdaust]

Agreed. Specifically for the raster option, it would be worth potentially having the internal functions use terra direction instead of data.table, e.g. as in https://github.com/bcgov/climr/issues/84#issue-1899148480