sPuReMD: re-enable SMALL_BOX_SUPPORT. Fix issue with far nbrs list being incorrectly generated for small boxes (missing nbr ID). Fix issue with ACKS2 matrix being incorrect for small boxes due to multiple non-zeros being added for each perioidic image of an atom (accumulate instead). Add missing ACKS2-specific energy and forces term. Clean-up far nbr list estimation and generation code for small boxes. Do not add far list struct entries unless the atom is below the specified cutoff.