modcc: AVX512 vectorisation backend (#154)

Basic features:

* Compile with -t avx512
* Automatically set up by CMake if USE_OPTIMIZED_KERNELS is on and VECTORIZE_TARGET is set to KNL 
* Generic SIMD printer that contacts a SIMD backend for emitting the actual SIMD intrinsics

Note: compilation for the avx512 target requires the Intel compiler.