Rewrite some stuff with kernex

[duxfrederic]

In the future consider using https://github.com/ASEM000/kernex

will try to rewrite our stuff with it, might look cleaner and be faster?

[aymgal]

If it leads to faster operations that would be great! Especially with big kernels. I don't understand exactly what are these "stencil" things, but it looks neat