Fix error in nonplanarcosseratshellenergy

Add a simple test with a grid containing one element testing if the energy is invariant of the number of grid refinements.

dune/gfe/nonplanarcosseratshellenergy.hh

@@ -228,8 +228,11 @@ energy(const typename Basis::LocalView& localView,
     aCovariant[2] = Dune::MatrixVector::crossProduct(aCovariant[0], aCovariant[1]);
     aCovariant[2] /= aCovariant[2].two_norm();
 
-    auto aContravariant = Dune::GFE::transpose(aCovariant);
-    aContravariant.invert();
+    auto aCovariantInv = aCovariant;
+    aCovariantInv.invert();
+    // The contravariant base vectors are the *columns* of the inverse of the covariant matrix
+    // To get an easier access to the columns, we use the transpose of the contravariant matrix
+    auto aContravariant = Dune::GFE::transpose(aCovariantInv);
 
     Dune::FieldMatrix<double,3,3> a(0);
     for (int alpha=0; alpha<gridDim; alpha++)