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.

Note that view limits are not part of the state of the camera. The idea is that you can set the limits once, and then when the camera is `reset()`, it uses these limits to set its parameters (i.e. state) to a reasonable initial view. 

I think the state of the current cameras can all  be boiled down to: center, scale_factor, fov, + rotations. (e.g. I don't think we need to incorporate speed/acceleration of the fly camera.)