adapted inverse_active_deformation to Rossi model

src/elasticity/materials/materials.hpp

@@ -127,9 +127,11 @@ namespace mat {
            + sqrt(gamma[0]) * (One - Product(Q[0], Q[0]));
       return AD;
     } else if (name == "Quarteroni2") {
-      AD = (1 / gamma[0]) * Product(Q[0], Q[0])
-           + scale_gamma * sqrt(gamma[0]) * Product(Q[1], Q[1])
-           + (sqrt(gamma[0]) / scale_gamma) * Product(Q[2], Q[2]);
+      if (1+gamma[0] < 0)
+	return One;
+      AD = (1 / (1+gamma[0])) * Product(Q[0], Q[0])
+           + scale_gamma * sqrt(1+gamma[0]) * Product(Q[1], Q[1])
+           + (sqrt(1+gamma[0]) / scale_gamma) * Product(Q[2], Q[2]);
       return AD;
     } else if (name == "") {
       AD = One - gamma[0] / (1 + gamma[0]) * Product(Q[0], Q[0])