ENH: triad: Add static identity triad representing orthonormal system

src/OpenFOAM/primitives/triad/triad.C

@@ -66,6 +66,13 @@ const triad triad::min
     vector(-VGREAT, -VGREAT, -VGREAT)
 );
 
+const triad triad::I
+(
+    vector(1, 0, 0),
+    vector(0, 1, 0),
+    vector(0, 0, 1)
+);
+
 const triad triad::unset
 (
     vector(VGREAT, VGREAT, VGREAT),

src/OpenFOAM/primitives/triad/triad.H

@@ -100,6 +100,7 @@ public:
         static const triad one;
         static const triad max;
         static const triad min;
+        static const triad I;
         static const triad unset;