Fix sign of demagnetizing energy in the right place

The previous commit changed the sign of the contribution of the volume
charges to the demagnetizing energy. It turns out the sign within the
expression was right to begin with. However, it relied on
Tet::interpolation(), which had the wrong sign. This error did not
affect the magnetization dynamics because that particular overload of
Tet::interpolation() is only used to compute the demagnetizing energy.

For sanity's sake, use the right sign in both places.

tetra.cpp

@@ -233,7 +233,7 @@ double Tet::demagEnergy(Eigen::Ref<Eigen::Matrix<double,DIM,NPI>> dudx,
 
     for (int npi = 0; npi < NPI; npi++)
         { dens[npi] = (dudx(0,npi) + dudy(1,npi) + dudz(2,npi)) * phi[npi]; }
-    return 0.5*mu0*Ms*weight.dot(dens);
+    return -0.5*mu0*Ms*weight.dot(dens);
     }
 
 double Tet::zeemanEnergy(Tetra::prm const &param, Eigen::Ref<Eigen::Vector3d> const Hext,

tetra.h

@@ -189,7 +189,7 @@ class Tet : public element<N,NPI>
         {
         Eigen::Matrix<double,N,1> scalar_nod;
         for (int i = 0; i < N; i++) scalar_nod(i) = getter(refNode[ind[i]]);
-        result = -scalar_nod.transpose() * eigen_a;
+        result = scalar_nod.transpose() * eigen_a;
         }
 
     /** interpolation for 3D vector field and a tensor : getter function is given as a parameter to