avx

Hi,

thank you very much for this cool library.

I noticed that simde_mm512_load_ps is missing while simde_mm512_loadu_ps is implemented. However, both seem to exist for Intel intrinsics (see screenshot). Maybe I oversaw one or maybe they do the same anyways?

Thanks
Lukas

![Screenshot from 2021-10-25 13-28-29](https://user-images.githubusercontent.com/14993470/138687358-aa6b531e-bdc

We have an implementation of Prewitt filter which locates in src/filtering.cpp file. We would like to add an implementation of the same filter for 16-bit unsigned integer images. Please use Image16Bit class for these purposes.