fix error in set.names()

[dfriend21]

set.names() wasn't working properly for SpatRasterDataset or SpatRasterCollection - here's a example testing set.names with the different Spat* classes:

library(terra)

# SpatRaster ----
r <- c(rast(), rast())
set.names(r, c("a", "b"))
names(r)
#> [1] "a" "b"

# SpatRasterDataset ----
r <- sds(rast(), rast())
set.names(r, c("a", "b"))
#> Warning in 1:nlyr(x): numerical expression has 2 elements: only the first used
#> Error in if (length(value) != nlyr(x)) {: the condition has length > 1
names(r)
#> [1] "" ""

# SpatRasterCollection ----
r <- sprc(rast(), rast())
set.names(r, c("a", "b"))
#> Warning in 1:nlyr(x): numerical expression has 2 elements: only the first used
#> Error in if (length(value) != nlyr(x)) {: the condition has length > 1
names(r)
#> [1] "" ""

# SpatVector ----
v <- vect(cbind(0,0), atts = data.frame(x = 1, y = 2))
set.names(v, c("a", "b"))
names(v)
#> [1] "a" "b"

.raster_names_check() was being used to check the names, which uses nlyr to make sure the length is correct - this was causing an error for a SpatRasterDataset and SpatRasterCollection. I made a more generic .names_check function that accepts a function (lengthfx) for determining the length of the object - and then for SpatRasterDataset and SpatRasterCollection I passed length to lengthfx. With these changes, the above example works for all classes.

[rhijmans]

Thank you, that looks very good.