First part of commit for redesigned SIMD module - namechanges.

This patch contains the bulk part of trivial name changes
in preparation for the new SIMD module.
- Add the gmx_simd_ prefix, and _r suffix for real simd data.
  This makes the module more generic, and we separate the
  gromacs SIMD layer from x86 implementation better.
- epi32 (intel name to indicate 64-bit var extended to 128 bits)
  has been changed to int32 in types and defines.
- Negated-fused-multiply-add nomenclature has been changed
  so "-a*b+c" is fnmsub. This is the standard on both Intel
  and AMD, where most people have SIMD coding experience.
  My apologies for first recommending Berk to use the opposite.
- GMX_CPU_ACCELERATION_ strings have been changed to GMX_SIMD_,
  both in the code and in CMake.
- Defines like GMX_X86_SSE2 that indicate instruction set
  availability have received the _OR_HIGHER suffix to clarify
  that those are cumulative in contrast to the SIMD setting,
  which only indicates the highest level. These will be moved
  away from config.h to the simd headers in the next patch.
- GMX_SIMD_WITDH_HERE has been changed to GMX_SIMD_REAL_WIDTH.

Change-Id: I7a2567f7ddaf8ecd95a4f3b1162bbf03342b2b63