Fix in mooneyrivlindensity

Square the determinant in the Ciarlet variant

dune/elasticity/materials/mooneyrivlindensity.hh

@@ -66,7 +66,7 @@ public:
        C = F^TF and B = FF^T have the same eigenvalues - we can use either one of them.
        
        There are three Mooney-Rivlin-Variants:
-       ciarlet: W(F) = mooneyrivlin_a*(normF)^2 + mooneyrivlin_b*(normFinv)^2*detF + mooneyrivlin_c*(detF)^2 -
+       ciarlet: W(F) = mooneyrivlin_a*(normF)^2 + mooneyrivlin_b*(normFinv)^2*detF^2 + mooneyrivlin_c*(detF)^2 -
                        ((dim-1)*mooneyrivlin_a + mooneyrivlin_b + 2*mooneyrivlin_c)*ln(detF)
        log:     W(F) = \sum_{i,j=0}^{i+j<=n} mooneyrivlin_ij * (I1 - 3)^i * (I2 - 3)^j + mooneyrivlin_k * ln(det(F))^2
        square:  W(F) = \sum_{i,j=0}^{i+j<=n} mooneyrivlin_ij * (I1 - 3)^i * (I2 - 3)^j + mooneyrivlin_k * 0.5 * (det(F) - 1)^2
@@ -99,7 +99,7 @@ public:
       gradientInverse.invert();
       field_type frobeinusNormFInverseSquared = gradientInverse.frobenius_norm2();
       using std::log;
-      return mooneyrivlin_a*frobeniusNormFsquared + mooneyrivlin_b*frobeinusNormFInverseSquared*detF + mooneyrivlin_c*detF*detF - ((dim-1)*mooneyrivlin_a + mooneyrivlin_b + 2*mooneyrivlin_c)*log(detF);
+      return mooneyrivlin_a*frobeniusNormFsquared + mooneyrivlin_b*frobeinusNormFInverseSquared*detF*detF + mooneyrivlin_c*detF*detF - ((dim-1)*mooneyrivlin_a + mooneyrivlin_b + 2*mooneyrivlin_c)*log(detF);
     }
     else
     {