WIP: Volume rendering

This PR implements volume rendering. It's basically a port of the technique that I use in Visvis.

* New Volume visual with two rendering styles (iso and mip). 
* Volume example in examples/scene/volume.py Try it, and use the keys to switch cameras etc.
* Refactored cameras a bit in general
* Refactored TurnTableCamera: no more mode and distance. You can just change the fov while getting the same zoom level. Setting fov to 0 means orthographic. Zooming is done by moving camera backward.
* New FlyCamera to fly around your data. 

This is about ready. There are more things to do, but that can be picked up in another PR. (I added todo items for this in volume.py)

I wish to do more with the camera's, but we should probably discuss this first. Will make a new issue for this.

> The beauty of the current system is that there is no need to specify distance. 

And the problem with it is that there is no _way_ to specify the distance. The scale factor might indeed be useful, but it should not come at the expense of making the cameras magical black boxes. I also suspect that the absence of a truly orthographic transform will be problematic in some situations due to fp issues.