Add tutorial topics

[danieljfarrell]

Some notes to self on documentation topics

1. Simulating large-scale scenes

Units can be a problem here as we find EPS_ZERO gotchas

2. Problem with linear background models

Useful short-hand but quite inefficient as ray bounce around inside for a long time

3. Importance of fitting absorption coefficient data

Reabsorption details are all captured in the tail of the distribution