ENH: provide a 'dimensioned' null-like constructor for dimensionedType

- The null constructor already creates a dimensionless Zero,
  but named "undefined".

  Provide an constructor for a dimensioned Zero,
  but named "0" for universal clarity to its value.

applications/test/dimensionedType/Test-dimensionedType.C

@@ -81,6 +81,10 @@ int main(int argc, char *argv[])
     }
 
 
+    Pout<< "zero scalar (time): " << dimensionedScalar(dimTime) << endl;
+    Pout<< "zero vector: " << dimensionedVector(dimLength) << endl;
+    Pout<< "zero tensor: " << dimensionedTensor(dimLength) << endl;
+
     Info<< "End\n" << endl;
 
     return 0;