add a blueNorm() function implementing the Blues's stable norm

algorithm. it is currently provided for experimentation
purpose only.

Eigen/src/Core/NumTraits.h

@@ -70,7 +70,9 @@ template<> struct NumTraits<float>
     HasFloatingPoint = 1,
     ReadCost = 1,
     AddCost = 1,
-    MulCost = 1
+    MulCost = 1,
+    Base = 2,
+    Mantissa = 23
   };
 };
 
@@ -83,7 +85,9 @@ template<> struct NumTraits<double>
     HasFloatingPoint = 1,
     ReadCost = 1,
     AddCost = 1,
-    MulCost = 1
+    MulCost = 1,
+    Base = 2,
+    Mantissa = 52
   };
 };