Bugfix: by error parameter L_c_ lost a square in a recent patch

This patch brings it back.

[[Imported from SVN: r9477]]

dune/gfe/cosseratenergystiffness.hh

@@ -365,9 +365,9 @@ public:
     RT curvatureEnergy(const Tensor3<field_type,3,3,3>& DR) const
     {
 #ifdef DONT_USE_CURL
-        return mu_ * std::pow(L_c_ * DR.frobenius_norm2(),q_/2.0);
+        return mu_ * std::pow(L_c_ * L_c_ * DR.frobenius_norm2(),q_/2.0);
 #else
-        return mu_ * std::pow(L_c_ * curl(DR).frobenius_norm2(),q_/2.0);
+        return mu_ * std::pow(L_c_ * L_c_ * curl(DR).frobenius_norm2(),q_/2.0);
 #endif
     }