Merge branch 'macro-vectorisation' into 'master'

Macro vectorisation

Created a macro called `WITH_VECTORIZATION` in `config.h` that enables vectorisation throughout the code. Replaced all occurrences of `VECTORIZE` with `WITH_VECTORIZATION` throughout the code. Replaced compiler macros, i.e `__AVX__`,`__MIC__` with platform defined macros: `HAVE_AVX`, `HAVE_AVX512_F`... in `vector.h`.

Added place holders for vectorised versions of force and density interactions for Gadget2 and Minimal SPH schemes. An error will be displayed at runtime if the code is compiled with vectorisation enabled and using either Gadget2 or Minimal SPH schemes.

See merge request !200