Add 3D SOM and examples

[StevenGuo42]

SOM with 3 dimensional map
examples: 3D color mapping, and mapping hand written digits to a 3D space

[JustGlowing]

hi @StevenGuo42, thanks for submitting this pull request. I'll have a look during the weekend.

[JustGlowing]

Dear @StevenGuo42,

I really appreciate your hard work but unfortunately I don't see a way to merge this pull request for the following reasons:

• The feature implemented doesn't allow the library to solve new problems and adds a parameter that is hard to optimize.
• It doubles the lines of code of the project and there are plenty of if-then statements to deal with specific cases. This decreases the maintainability of the code (and possibly the performances as there's an if-then in the main training loop).

There's definitely value in what you have done but I don't see it fitting well in the current shape of the repository. However, I would be happy to have an example of how to use a 3D som if you find a way to keep your work self contained in a notebook.

[StevenGuo42]

Hello @JustGlowing
Thanks for the feedback. I got quite busy recently and that will take me a while to finish.