documentation addtion for buffland

[plantarum]

Since buffland is provided explicitly for use as a reference with cc_sea(), I expected the following would work:

data("buffland")
cc_sea(data, ref = buffland)

This fails because buffland is not class "SpatVector", but is actually a "PackedSpatVector".

That's easy enough to do once you know how. Would it be possible to either:

1. provide buffland as a "SpatVector" so this conversion isn't necessary
2. add a short example of using it in the documentation for buffland or cc_sea? e.g.,

data("buffland")
buffland <- unwrap(buffland)
cc_sea(data, ref = buffland)

Otherwise, this step is only mentioned in the vignette, but not in any of the man pages for the associated functions.

Thanks!