Add test for Verlet cut-off scheme with free energies

complex/nbnxn-free-energy is a copy of complex/fe_test with minor
changes noted in the README there. This should be an effective
test of FEP list build, exclusion of FE pairs from the normal lists,
and the PME grid correction.

Change-Id: Iac69b4e92e2d2de48ee50a6b77b6f7db56dabfbe