Fix LJPME Lorentz-Berthelot combination rules

Christian found a bug in the way we evaluate
arithmetic combination rules for LJ-PME reciprocal
space. With this change, both group and verlet kernels
produce virtually identical total energies for both
types of reciprocal-space combination rules. It does
not affect the (default) geometric LJPME case.

Change-Id: I4bb0de1df9588dce3ca94e39a370a0b994b25b53