Correct the sign of the Neumann and volume forces to make it consistent...

Correct the sign of the Neumann and volume forces to make it consistent throughout dune-gfe and dune-elasticity

In the energy file, the energies are added. The Neumann and volume forces get the correct sign in the actual program file by mulitplying with (-homotopyParameter).

@osander: Can you have a look please? Thanks!