Proposal: add abstract types ImageAlgorithm and ImageFilter

I propose to add two abstract types

abstract type AbstractImageAlgorithm end
abstract type AbstractImageFilter<: AbstractImageAlgorithm end

where filters are algorithms whose input and output are both images, so that they can be stacked together, i.e., new_img = old_img |> filter1 |> filter2 |> ...|> filterN.

These two types serve in two ways: as the root of the unified algorithms hierarchy system, and as a placeholder in ImagesAPI.jl

use case:

# ImagesAPI.jl
remove_noise(img, ::AbstractImageFilter) = img

# ImageNoise.jl
using ImageAPIs: AbstractImageFilter, denoise

abstract type AbstractImageDenoiseFilter <: AbstractImageFilter end
struct BlockMatching3dFiltering <: AbstractImageDenoiseFilter end
remove_noise(img::GenericImage, f::AbstractImageDenoiseFilter) = f(img)

function (f::BlockMatching3dFiltering)(img::GenericImage)
...
end

related issues:

https://github.com/JuliaImages/Images.jl/issues/772
https://github.com/zygmuntszpak/ImageBinarization.jl/issues/23
https://github.com/zygmuntszpak/ImageBinarization.jl/issues/26

cc: @zygmuntszpak

Start by reading the existing ImagesAPI.jl definitions and the related issues linked in the proposal. Compare the proposed AbstractImageAlgorithm and AbstractImageFilter hierarchy with the demonstrated ImageNoise.jl use case, then confirm that the resulting API supports the stated filter composition and dispatch examples.