This systems test the non-bonded NxN kernels when two atoms are on top of
each other. With reaction-field and PME (not tested) excluded atoms at
distance zero should have a non-zero electrostatic energy and zero force.
This system also tests LJ parameters not obeing combination rules
(only relevant for the x86 SIMD NxN kernels, which are comb.rule specific).
This systems also tests periodic interactions within one cluster,
as there are only 4 atoms.
