Fancier transparencies in Volume.draw

[vischia]

Currently, the passive layers are painted as four opaque (alpha=1) blocks. It would be nice if the back walls were opaque and the front ones were semitransparent.

This requires passing, in Volume.draw, the tensors of vertices of the walls as two separate groups with different alpha.

Bonus feature: when the semitransparency is implemented, draw additional walls to mark the individual voxels (not sure if this is confusing to the eye, but I have seen nice examples around so it should be doable).

[vischia]

Questionable. "front" and "back" walls are based on a specific view. If the users uses the %matplotlib notebook magic to rotate the figure, this functionality can make things ugly.

It's better to always have all the passive layer walls transparent (but more opaque than detector layers), and then matplotlib will take care of the transparency in rotations.