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.

Unless you want a more "regular" perspective camera that flies around your center point, for which modifying fov *does* give an impression of zooming. But I suspect in most cases the new system is more useful.

I like how the fov does not affect the zoom level (at least not at the center), such that the ortographic and perspective "modes" give equal zoom level. I took these ideas from visvis, where Robert Scholl implemented this. Credit goes to him.