joeroe/rpaleoclim: rpaleoclim 1.0.0

Added an introductory vignette Migrated from terra to raster: paleoclim() and load_paleoclim() now return a terra::SpatRaster instead of a raster::raster. paleoclim(as = "raster") and load_paleoclim(as = "raster") are provided for backwards compatibility. These require raster to be installed and will be removed in future versions. The region argument of paleoclim() now expects a terra::ext instead of a raster::extent. extent is coercible to ext, so this shouldn't break any existing code. Fixed name of period "mis19" (was "mis"