Energy Calculation
Energy calculations in takeTimeStep() use Kokkos::atomic_add().
Currently, temporal values are used to sum energy values to check the values in unittest_EnergiesMeumappsSS.cpp. It needs to be updated later.
on GPU, _elDisp and _elEner shows an error. They are commented out in this MR.
Calculations for el_grad and el_en are required to be updated accordingly.
All tests passes on CPU and GPU.