ModSim ass4 taddeus: Added usage to comparison shell script.

modsim/ass4_taddeus/vibstring/diff.sh

 #!/bin/bash
-args=${@:2:$#}
-./seq $args > out_seq.txt
-#mpirun -np $1 ./par $args
+if [ $# -lt 7 ]; then
+    echo "Usage: $0 NUM_PROCESSES INIT_METHOD TIME_STEPS LENGTH DX TAU N|XP"
+    exit 1
+fi
+./seq ${@:2:$#} > out_seq.txt
 bash par.sh $@
 python combine.py $1 > out_par.txt
 diff out_{seq,par}.txt