Add package extension for visualization of colorings

[adrhill]

Add an un-exported show_colors function as well as tests and documentation for it.
The function has some options for customization:

    show_colors(result)

Return an image visualizing an [`AbstractColoringResult`](@ref).

!!! warning
    Using this function requires loading [Images.jl](https://github.com/JuliaImages/Images.jl).

## Keyword arguments
* `colorscheme`: colors used for non-zero matrix entries. 
* `background`: color used for zero matrix entries and padding. Defaults to a transparent background.
* `scale::Int`: scale the size of matrix entries to `scale × scale` pixels. Defaults to `1`. 
* `padding::Int`: padding between matrix entries. Defaults to `0`. 

Currently the function errors when the matrix-coloring has more colors than the provided colorscheme. I can add some color cycling to circumvent this if desired.

Edit: turned this from an Images.jl extension to a ColorTypes.jl extension to make it more light-weight.

[adrhill]

The code is written to make it as easy as possible to extend _show_colors! on future AbstractColoringResult types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7146a7d) to head (0cbb367).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #149    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           12        13     +1     
  Lines          884      1010   +126     
==========================================
+ Hits           884      1010   +126     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[gdalle]

Thanks for this very pretty contribution!

[adrhill]

This is what the new doc page looks like:

Visualization · SparseMatrixColorings.jl.pdf

[adrhill]

Excited for this!

[gdalle]

Awesome!

[adrhill]

Would be nice to have a release tagged for this! :)

[gdalle]

#150

[amontoison]

Thanks @adrhill for working on that!

[adrhill]

Check it out in action! :)
https://gdalle.github.io/SparseMatrixColorings.jl/stable/vis/