[kernel-generation] Investigate alternatives for types for kernel generation

[simmplecoder]

At the moment, simple kernels (normalized and unnormalized mean, Gaussian blur) use boost::gil::gray32f_view_t, but that might not be the only, or best, solution.

References

• https://lists.boost.org/boost-gil/2019/07/0273.php
• @mloskot's suggested in https://github.com/boostorg/gil/pull/342#pullrequestreview-264528946 to make it compatible with the existing kernel_2d types