Use the Hapke parameters to correctly show light irradiance on airless regolith surfaces like the moons

[andreasfranke1]

See https://en.wikipedia.org/wiki/Hapke_parameters for details and https://aalberik.itch.io/demeter/devlog/1459430/beyond-the-lambertian-sphere-implementing-hapke-brdf-and-planetary-scattering for a related game to implement them.
Lambertian BRDFs incorrectly assume light irradiance to be uniform, so when looking at the full moon in the application, its edges darken:

However, it should rather look like this:

[charliewhitfield]

In master already, together with darkened edges for Venus and Titan. Will be released in v0.2.1 in the next week or so...

See section about this in the new PHOTOMETRIC_MODEL.md.

[andreasfranke1]

Cool, thanks!

[charliewhitfield]

This is in v0.2.2. It's correctly flattened for most bodies and the reverse (darker edges) for Venus and Titan.