Implemented nbnxn LJ switch functions

The nbnxn C reference and SIMD kernels now support force and
potential switching, which are selected through vdw_modifier.
Both use analytical functions, since tables are expensive with SIMD.
Made the nbnxn kernel naming consistent with the group scheme.
Corresponding CUDA kernels will be added soon.

Dispersion correction always needs table generation, which had always
been a fortuitous side effect of something else, until this patch.

Change-Id: I52a1cda5ec7c3bdce6f3f2735ff7b13303ea5880