Update demos to support latest changes in Raysect and Matplotlib

[vsnever]

A few demos throw errors due to incompatibility with latest versions of Raysect or Matplotlib:

• optics/logging_trajectories.py The logging ray now logs the Intersection objects instead of Point3D,
• optics/etendue_of_pinhole.py The arguments nonposx and nonposy of Matplotlib's set_xscale() and set_yscale() methods were replaced with nonpositive a few releases ago,
• materials/volume.py The Spectrum.samples is now read-only, so updating it with spectrum.samples += ... no longer works and must be replaced with spectrum.samples[:] += ...,
• materials/modifiers/transform.py Same as for materials/volume.py,
• materials/volume.py Missing ioff() and show().

I've already fixed these, and will make a PR now.

[vsnever]

Also, materials/tetrahedra_mesh_emission.py fails to run with the latest version of Py3DViewer, but a bug is on their side, https://github.com/cg3hci/py3DViewer/issues/11.

Update: Fixed.