Add output of lux landing on a surface, projected onto an output plane.

[sammorrell]

We need output of illuminance, in units of lux, in the model. Most of the ecological and professional work that is based around this research makes use of illuminance, in the units of lux. For this we need to complete several tasks:

• Integrate the new plane output into the model.
• Add a new attribute that can rasterise photons using different methods into an output plane.
• Implement a module containing (at least) the photopic luminous efficacy function.
• Integrate into MCRT