decorana: require and check that data are numeric

character data (or vector code) starting with " " is regarded < 0,
and give error of negative data. This can be behind the confusion
in https://stackoverflow.com/questions/78666646/

R/decorana.R

@@ -8,6 +8,8 @@
     ZEROEIG <- 1e-7 # same limit as in the C function do_decorana
     ## data
     veg <- as.matrix(veg)
+    if (!is.numeric(veg))
+        stop("data must be numeric (no factors nor characters)")
     if (any(veg < 0))
         stop("'decorana' cannot handle negative data entries")
     ## optional data transformation