advertise `ccolor` in writing generic code

Functions in JuliaImages should accept both numerical array AbstractArray{<:Number} and colorant array Abstract{<:Colorant} if possible.

It is not uncommon to infer the return eltype in the beginning of the algorithm so as to pre-allocate array. I previously have two versions of codes to achieve this. The first version is:

function foo(img::AbstractArray{T}) where T<:Number
    CT = RGB{floattype(T)}
    ...
end

function foo(img::AbstractArray{T}) where T<:Colorant
    CT = RGB{floattype(eltype(T))}
    ...
end

Then one day I found out that eltype(Float32) == Float32, which make it possible to merge it into one method:

function foo(img::AbstractArray{T}) where T<:Number
    CT = RGB{floattype(eltype(T))}
    ...
end

As @kimikage pointed it out in https://github.com/JuliaGraphics/ColorTypes.jl/issues/201, ccolor does a similar thing more reliably (I assume so?):

function foo(img::AbstractArray{T}) where T<:Number
    CT = ccolor(RGB, floattype(T))
    ...
end

I feel this worth documenting somewhere in the tutorials; otherwise, it's like a missing spell and people repeatedly reinvent it.

Start by locating the JuliaImages tutorials and reviewing where generic image-processing code is documented. Add a tutorial explanation of using ccolor with floattype for both numerical and colorant arrays, using the examples in this issue. Done means the pattern is documented clearly enough to prevent repeated reimplementation.