Adjust Mooney-Rivlin energy in the Ciarlet case s.t. W(I)=0

This is done by subtracting "dim" or "1" at the given terms. Now it is consistent with the other 2 cases of MR energy.

dune/elasticity/materials/mooneyrivlindensity.hh

@@ -66,9 +66,11 @@ 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^2 + mooneyrivlin_c*(detF)^2 -
-                       (2*mooneyrivlin_a + 4*mooneyrivlin_b + 2*mooneyrivlin_c)*ln(detF),
-                       where the last term is chosen s.t. W( t*I ) is minimal for t=1
+       ciarlet: W(F) = mooneyrivlin_a * [ (normF)^2 - dim ]              # -dim for w(I) = 0
+                     + mooneyrivlin_b * [ (normFinv)^2*detF^2 - dim ]    # -dim for w(I) = 0
+                     + mooneyrivlin_c * [ (detF)^2 - 1 ]                 # -1   for W(I) = 0
+                     + (2*mooneyrivlin_a + 4*mooneyrivlin_b + 2*mooneyrivlin_c)*ln(detF),
+                       where the last term is chosen s.t. W( t*I ) is minimal for t=1.
        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
 
@@ -100,7 +102,9 @@ public:
       gradientInverse.invert();
       field_type frobeinusNormFInverseSquared = gradientInverse.frobenius_norm2();
       using std::log;
-      return mooneyrivlin_a*frobeniusNormFsquared + mooneyrivlin_b*frobeinusNormFInverseSquared*detF*detF + mooneyrivlin_c*detF*detF
+      return   mooneyrivlin_a * ( frobeniusNormFsquared - dim )
+             + mooneyrivlin_b * ( frobeinusNormFInverseSquared*detF*detF - dim )
+             + mooneyrivlin_c * ( detF*detF - 1.0 )
              - (2.0*mooneyrivlin_a + 4.0*mooneyrivlin_b + 2.0*mooneyrivlin_c)*log(detF);
     }
     else