New n-dimensional scatter plot example with GPU-based smooth projections

Not ready for merge, but you can already try it!

Things to be done:

* [ ] minor simplification in `OrthogonalPath`
* [ ] support for n-dimensional data points (instead of 4 dimensions max here), related to #390
* [x] drop dependency on sklearn by including the dataset in vispy (#389).

Close #389.

I think I prefer to be explicit here, just to be sure... Float64 arrays may lead to subtle and hard-to-debug bugs, so I'd say it's good practice to do it even when it's not strictly necessary.