SSE2 single-precision kernels

SSE2 single-precision kernels

New single-precision x86 kernels rewritten to use intrinsics
instead of raw assembly. SSE2 is now our lowest-supported
accelerated instruction set on x86. We generate kernels for
the same interaction choices as the plain c kernels, but
have not enabled Buckingham interactions SSE kernels since
that would generate a whole lot more kernels that would only
be use in rare circumstances. The neighborsearching routine
has also been updated to optionally generate lists that
are padded up to the simd width, which is set by each
kernel when we initialize the neighborlist.

Change-Id: Ic51078e10e7f1f29af3d33e256177806ec208646