bugfix multi_class_samplig() and plot_results()

two new bugs due to R version change:

1. the condition if (class(ref) == "SpatialPointsDataFrame" in multi_Class_Sampling() does not work if there are two classes for the reference argument e.g. class(reference) "matrix" "array"
2. the function that checks length(step*.kml) == length(step*.tif) in plot_results() does not work if the *.tif.aux.xml is also written to disk int the new raster or terra version