A slightly different take on#63 that uses an option (currently calleduse_cache) to switch on behavior to deal with#58 instead of adding an additionaltar_terra_*() function.

Whenuse_cache isTRUE, the raster is written withterra::wrapCache() tocache_dir (default_geotargets/) and the output (aPackedSpatRaster object) is saved to the targets store withsaveRDS(). This allows 'sidecar' files to exist and be read back in by the read function (terra::unwrap(readRDS(path))).

I'd love any feedback onnaming things andwhere to document this. E.g. I'm not sure that "cache" is really a good/complete descriptor for what's going on here.use_external_cache?use_raster_cache?use_proxy_cache?

Things to do in other PRs before this one is ready:

• update parameter documentation to customize some currently inherited documentation now that help files fortar_target() andtar_target_raw() have been merged intargets (Update parameter documentation #107,custom documentation for name, command, and pattern params #108)

Things to do in this PR before it is ready:

• document()
• Make sure management of the_geotargets/ cache is user-friendly. E.g. when that cache is destroyed (by ageotargets function), the corresponding targets are also invalidated.
• possibly implement something liketar_prune() but for the_geotargets/ cache.
• testing and/or documentation that this probably doesn't work with cloud storage