Add example demonstrating spike-based Sudoku solver

[krachbumm3nte]

A NEST implementation of Sudoku as a constraint satisfaction problem based on (https://github.com/SpiNNakerManchester/IntroLab/tree/master/sudoku)

[heplesser]

@krachbumm3nte Thank you for porting this exciting example to NEST. I noticed that some of your scripts reference S Furber and A Rowley as authors. I am therefore wondering to what degree your code is written from scratch and to which degree it is modified from existing code by Furber/Rowley? I believe their original implementation was in PyNN.

We need to make sure we do the right thing with respect to copyright and licensing.

I enter this comment as "request changes" to ensure that we won't merge before we have clarified this point.

[krachbumm3nte]

@heplesser The actual code shares very little resemblance to the original implementation. The parts I copied are the connection statistics, neuron parameters and the exemplary sudoku puzzles (helpers.py/get_puzzle()).
I also contacted both of the original authors and got permission to publish my version with their names attached where relevant.

[jessica-mitchell]

Thanks for this contribution @krachbumm3nte, really cool example! I noted a few things to fix up with the formatting and such. Please make sure to fix conflicts first before applying reviewer changes.

[jessica-mitchell]

Thanks @krachbumm3nte. I have one small formatting adjustment, but I will approve. I would like to make the gif more visible, but I think it makes sense to do that with / after #2289.

[gtrensch]

Thank you for porting this example to NEST. 👍
I approve the PR, but also would have a minor comment: While testing the example, I went into a dependency issue related to the Python imaging library PLI. Discussed this with @terhorstd, and we would like to replace PLI with a matplotlib solution at a later time. We opened #2441 to follow up on this.

The point in question was answered, re-request of review was unanswered, replaced reviewer.