Feature/max projection volume rendering less passes

[seankmartin]

Min and max projection VR modes with picking.

Adds:

1. Min and Max projection modes to the volume rendering. Both of these work by computing the min/max over rays within single chunks in the shader, and then combining across chunks via depth testing.
2. Experimental picking works in Min and Max mode because we know the depth of the max/min sample.
3. First pass documentation about volume rendering.

Experimental change, open to thoughts:

Moved the axis helper visualisation to draw after volume rendering, so that it draws on top of it. Previously the axis helper was getting occluded in high-opacity volume rendering.