implemented plain-C SIMD macros for reference

This is mainly code reorganization.
Adds reference plain-C, slow, arbitrary width SIMD for testing.
Adds FMA for gmx_calc_rsq_pr.
Adds generic SIMD acceleration (also AVX or double) for pme solve.
Moved SIMD vector operations to gmx_simd_vec.h
The math functions invsqrt, inv, pmecorrF and pmecorrV have been
copied from the x86 specific single/double files to generic files
using the SIMD macros from gmx_simd_macros.h.
Moved all architecture specific nbnxn_kernel_simd_utils code to
separate files for each SIMD architecture and replaced all macros
by inline functions.
The SIMD reference nbnxn 2xnn kernels now support 16-wide SIMD.
Adds FMA for in nbnxn kernels for calc_rsq and Coulomb forces.

Refs #1173

Change-Id: Ieda78cc3bcb499e8c17ef8ef539c49cbc2d6d74d