Allow source in tissue

[JLBegin]

Changes

• You can now set a world material when creating a Scene (instead of later during propagation)
• Source can be positioned anywhere in the scene. Photon context will be automatically initialized with the correct environment.
• Added simple source graphic (shere) showed by default on Stats 3D displays

Implementation details

• Moved world material context out of source.propagate and into Scene
• Source environment is determined when starting propagation by asking the scene what is the environment at the source position (uses a special algo when dealing with stacks).
• Stats now adds the source photonCount to a solid's total input energy when the source is contained in it.

Example

Showing a few graphs from rayscattering/example.py with source positioned at Vector(0, 0, 0.3).

Report of solid 'frontLayer'
  Absorbance: 42.4% (43.2% of total power)
  Absorbance + Transmittance: 100.0%
    Transmittance at 'interface1': 56.1%
    Transmittance at 'front': 0.9%
    Transmittance at 'top': 0.2%
    Transmittance at 'left': 0.2%
    Transmittance at 'bottom': 0.1%
    Transmittance at 'right': 0.2%
Report of solid 'middleLayer'
  Absorbance: 51.8% (30.2% of total power)
  Absorbance + Transmittance: 100.0%
    Transmittance at 'interface1': 3.2%
    Transmittance at 'bottom': 0.3%
    Transmittance at 'interface0': 44.0%
    Transmittance at 'right': 0.3%
    Transmittance at 'left': 0.2%
    Transmittance at 'top': 0.2%
Report of solid 'backLayer'
  Absorbance: 67.9% (17.4% of total power)
  Absorbance + Transmittance: 100.0%
    Transmittance at 'interface0': 4.9%
    Transmittance at 'back': 23.2%
    Transmittance at 'top': 0.8%
    Transmittance at 'right': 0.9%
    Transmittance at 'bottom': 1.2%
    Transmittance at 'left': 1.2%