projectRasterForLeaflet should support method="ngb"

[adrfantini]

The raster package supports nearest neighbour projection in projectRaster, by issuing method="ngb". This should be possible in projectRasterForLeaflet too - it's useful for a number of different datasets.

[tim-salabim]

I totally agree that there needs to be an option for re-projecting factorial rasters using nearest neighbor. The linear interpolation of the bilinear re-projection (the default in raster::projectRaster) produces floating point values which, in addition to being false values for factorial raster layers, also mess up the legend produced with leaflet::colorFactor.

In mapview we actually do the raster projection outside the leaflet::addRasterImage call like this (lines 178 - 187 in https://github.com/environmentalinformatics-marburg/mapview/blob/master/R/leafletControls.R):

is.fact <- raster::is.factor(x)[1]
if (is.fact) {
    x <- raster::projectRaster(
      x, raster::projectExtent(x, crs = sp::CRS(wmcrs)),
      method = "ngb")
    x <- raster::as.factor(x)
  } else {
    x <- raster::projectRaster(
      x, raster::projectExtent(x, crs = sp::CRS(wmcrs)),
      method = "bilinear")
  }

The user still needs to make sure that their raster is of class factor (as having integer values does not necessary mean factorial, hence the explicit call to raster::as.factor after the re-projection), but that can be easily done with raster::as.factor