Add rain field to visualizer

[goord]

The rain field (qr) is an extra 3d field that can be visualized within zarrviz. It is sparse and can be displayed together with the clouds. Instead of the intricate physics-based rendering for the clouds, we can use a simple transfer function to render it (still volume rendering though). For the rendering, I see two possiblities

1. We add it to the cloud volume shader. The benefit is that this is possibly less work to implement, it is possibly less heavy on the GPU when looking at clouds+rain. It could also give less issues for visbility blocking.
2. We implement it as a separate shader material. This is the more modular approach.

Combining ql and qr will be a challenge because they have a different vertical extent.