createComparableCDL uses a base index within a raster list, and sets all other
raster images within the list to the same resolution, projection, and extent. The
raster function resample is used to tranform raster images, therefore this
function may be quite slow without tuning.
if (FALSE) {
# download multiple years of Iowa Datar <- getCDL('iowa',c(2006,2010))
# resample based on the 2006r2 <- createComparableCDL(r,baseIndex=1)
}