Add irradiance meter to hollow sphere particle tracer scenes

Feature-Start: particle-tracer-sensor-sampling
Add particle tracer version of disk irradiance scene
Add particle tracer version of square irradiance scene
Add particle tracer version of Cornell box scene
Add ParticleTracerSamplingMethod type
Add SamplePixelPositionAndWeFromPoint() method to Sensor

Write non-trivial implementations for IrradianceMeter, FluxMeter, and
PerspectiveCamera.
Add Light methods to sample/compute spatial and directional Le seperately

Also implement them for DiffuseAreaLight.
Implement ParticleTracer sensor sampling method

Use that method in all particle tracer scenes.

Feature-End: particle-tracer-sensor-sampling