Append shapefile center

Include a test so shapefile center is skipped if not present. The
triplet of shapefiles has possibly become a collection of shapefiles
since commit 4e2ab0a6.

shp_tr_append.sh

@@ -6,17 +6,20 @@
 
 if (($# != 2))
 then
-    echo "Usage: shp_tr_append.sh from_triplet to_triplet"
+    echo "Usage: shp_coll_append.sh from_coll to_coll"
     exit 1
 fi
 
 set -e
 ##set -x
 
-for my_base in extremum outermost_contour max_speed_contour
+for my_base in extremum outermost_contour max_speed_contour center
 do
-    shpcat $1/$my_base $2/$my_base
-    dbfcat $1/$my_base $2/$my_base
+    if [[ -f $1/$my_base.shp ]]
+    then
+	shpcat $1/$my_base $2/$my_base
+	dbfcat $1/$my_base $2/$my_base
+    fi
 done
 
 i2=`tail -1 $2/ishape_last.txt`