ENH: Adding -overwrite option to mirrorMesh and update tutorial

applications/utilities/mesh/manipulation/mirrorMesh/mirrorMesh.C

@@ -37,15 +37,24 @@ using namespace Foam;
 
 int main(int argc, char *argv[])
 {
+
+#   include "addOverwriteOption.H"
 #   include "setRootCase.H"
 #   include "createTime.H"
 
+    const bool overwrite = args.optionFound("overwrite");
+
+    if (!overwrite)
+    {
+        runTime++;
+    }
+
     mirrorFvMesh mesh
     (
         IOobject
         (
             mirrorFvMesh::defaultRegion,
-            runTime.timeName(),
+            runTime.constant(),
             runTime
         )
     );