Add ncells argument for global(x, fun=quantile)

[ailich]

In terra to get quantiles across cells for a layer you can use terra::global(x, fun=quantile). In raster this was done using raster::quantile. For large rasters this can take a long time or fail since I believe determining quantiles requires reading all values into memory. raster::quantile had an ncells argument to allow for a random sample of a specified size to be used instead. Perhaps this could be added if quantile is used as the function, or perhaps even better as an argument to global more generally.

[rhijmans]

Thanks. global has a new argument "maxcell"