quadtree(): add minimal support for SpatRaster input #7

[brownag]

Hi! Thanks for this great package.

I've switched essentially all my raster workflows to terra now and it would be convenient if quadtree() accepted the SpatRaster directly. So here is a 1-liner that adds support for SpatRaster inputs RE: #7.

I am not sure whether you have plans to rewrite internals that currently use raster with terra (which would of course be a bigger job). I would be happy to contribute PR(s) that front if you want to try and go that direction.

Otherwise something like this PR should cover most use cases; minimally allowing SpatRasters to be converted to RasterLayer, then to Quadtree.

[dfriend21]

Thanks! I've also switched to terra, so I agree that it would great to switch all of the raster-related functionality to terra. This'll at least serve as a stop-gap measure.

I'd love to get it switched over. However, this project isn't a central part of my work anymore, so any updates to it will have to happen on my free time - so unfortunately I can't guarantee promptness on this front. So any PRs concerning this issue (or any others) would be greatly appreciated!