[Math] Added a function to calculate the norm of deviatoric vector

MathLib/KelvinVector-impl.h

@@ -7,6 +7,8 @@
  *
  */
 
+#include <cmath>
+
 namespace MathLib
 {
 namespace KelvinVector
@@ -20,6 +22,13 @@ double Invariants<KelvinVectorSize>::equivalentStress(
     return std::sqrt(3 * J2(deviatoric_v));
 }
 
+template <int KelvinVectorSize>
+double Invariants<KelvinVectorSize>::Norm(
+    Eigen::Matrix<double, KelvinVectorSize, 1> const& deviatoric_v)
+{
+    return std::sqrt(deviatoric_v.transpose() * deviatoric_v);
+}
+
 template <int KelvinVectorSize>
 double Invariants<KelvinVectorSize>::J2(
     Eigen::Matrix<double, KelvinVectorSize, 1> const& deviatoric_v)

MathLib/KelvinVector.h

@@ -87,6 +87,10 @@ struct Invariants final
     static double equivalentStress(
         Eigen::Matrix<double, KelvinVectorSize, 1> const& deviatoric_v);
 
+    /// Get the norm of the deviatoric stress.
+    static double Norm(
+        Eigen::Matrix<double, KelvinVectorSize, 1> const& deviatoric_v);
+
     /// Second invariant of deviatoric tensor.
     /// \note The input vector must have trace equal zero.
     static double J2(