SIMD implementation of ConvertToGrayScale() function

[ihhub]

We do not have SIMD (SSE, AVX or NEON) implementation of ConvertToGrayScale() function. For the person who is willing to add such code please take a look into an implementation of the function inside image_function.cpp and the structure which is used for SIMD code inside image_function_simd.cpp. Please do not forget to add unit test for newly created function(s) inside unit_test_image_function.cpp (it's just one line code via macros call).

[ihhub]

Hi @0x72D0 would you mind to work on this issue?

[0x72D0]

Hi @ihhub I could jump on this after the avx512 implementation

[ihhub]

Thanks a lot!