Usage message for run_sims

run_sims.in

@@ -2,6 +2,11 @@
 
 seed="$1"
 
+if [ "$seed" == "" ]; then
+    echo "Usage: $0 seed"
+    exit 1
+fi
+
 # Uncomment the ones you want to run.  Note that each one takes only one
 # core, but a nontrivial amount of RAM (estimated in the line above).