Improve SLIC

* arbitrary channel combinations possible (not necessarily 3-channel images)
* improved speed by cythonized cluster center computation and only using necessary channels (for grayscale images I see a speed improvement of 4x)

I am highly recommending to revert some of the multi-channel magic. For me it was very confusing...

We must at least improve the documentation about the behavior of sigma and the multichannel parameters.

At the moment one of the test cases is failing, I have not had time to look into it.