Fixed free energy with LJ-PME

First, Only the normal LJ part of the potential was
shifted for LJ-PME with potential-shift
modifier in the free energy kernel,
and not the Ewald component. This had an effect
of roughly 0.05% on the total energy. Second,
Berk found that the verlet nbnxn SIMD LJ-PME kernel
had an issue that caused the potential and force
to be slightly off. With this patch, both
potential and force match the non-free-energy
kernel perfectly with at lambda=0.

Change-Id: I4bc53ce1bdb96ba06656b86470dbf8dbd1e81972