STYLE: use shell 'command' instead of 'which' in tutorials

tutorials/incompressible/pimpleFoam/laminar/planarPoiseuille/validation/createGraph

@@ -3,11 +3,11 @@
 tail -n +4 ../postProcessing/probes/0/U  | \
     tr -s " " | tr -d '(' | cut -d " " -f2-3 > ../Numerical.dat
 
-if ! which gnuplot > /dev/null 2>&1
-then
-    echo "gnuplot not found - skipping graph creation" >&2
+# Test if gnuplot exists on the system
+command -v gnuplot >/dev/null 2>&1 || {
+    echo "gnuplot not found - skipping graph creation" 1>&2
     exit 1
-fi
+}
 
 gnuplot<<EOF
     set terminal postscript eps color enhanced "Helvetica,20"