Fixed bug in the new switch-modified interactions in C kernels

Fixed bug in the new switch-modified interactions in C kernels

Fixed an error in the switch application formula, and another
table-related bug for PME interactions using the switch modifier
just introduced when only forces were calculated. This will not
have affected any other code unless you dug in the source and
tested the new switch modifiers the last 12h.

Change-Id: Ie323f45f2ce60ff1aacc2cccd9b395826f39cdf8