Skip completed tile indices in makeTiles()

[graymartin]

I am currently working on a project where I am taking a large stack of high-resolution rasters and saving the stack to a network share for distributed computation. I've tried using makeTiles() for this purpose and it is working extremely well. However, I did think of a potential quality-of-life improvement for the function.

Since writing to the network share can take a long time and has the potential to fail, it would be super helpful to have the ability to skip a certain number of tiles or select specific tile indices to run with this function. That way, I don't lose as much progress if the run fails because of a connection error or some other issue. Alternatively, it would be nice to be able to ignore/skip raster files that are already in the output directory.

I'm not sure how to go about adding this--I am not very experienced with coding in C++ and from what I can tell, makeTiles() calls a C++ function in raster_methods.cpp. If it seems like a good addition and wouldn't be too hard to add, it would be awesome to see this feature in a new version of terra!

[rhijmans]

Thank you for bringing this up and for your PR. I am proposing a somewhat different approach. With the changes I made, existing tiles are skipped if overwrite=FALSE (the default).

[graymartin]

Thank you so much! I like this approach a lot and appreciate the help.