Pass down shapes to sensors

Feature-Start: radiometers
Add PointShape class and use it to remove RadianceMeter's origin field
Make output of RadianceMeter more descriptive
Pull out mean/variance calculations into their own class
Add uniformSampleHemisphere and use it in DiffuseMaterial
Add ability to pass data around with a sensor-generated ray
Add IrradianceMeter sensor
Pull out shape-sampling functionality into shapeSet type
Pass down samples to Distribution1D directly
Pass samples directly to shapes
Rename SensorSampleRange to SensorSampleConfig
Add fields for additional 1d/2d samples in SensorSampleConfig
Add SampleSurface function to shapeSet
Add pSurfaceEpsilon return value to shape sampling methods
Add FluxMeter sensor

Feature-End: radiometers
Add Disk shape and Cornell box scene with disk light

Feature-Start: test-illuminance-scenes
Add disk irradiance scene with a known irradiance value
Add square irrandiance scene with a known irradiance value

Feature-End: test-illuminance-scenes