Sun position algorithm

[mororo250]

Draft of sun position implementation.

[dictoon]

Very partial and superficial first review!

[dictoon]

Hi,

@LZaw and I had another look at this PR. We feel that a few things need to be done in order for it to be merged.

In particular, the SunPositioner class presents itself as an appleseed entity (i.e. it inherits from renderer::Entity, etc.) but it doesn't really make sense.

Here is what we suggest:

1. Don't make SunPositioner inherit from Entity
2. Don't tag the class with APPLESEED_DLLSYMBOL
3. Remove the constructor, pass arguments natively
4. Remove the release() method
5. Remove the private implementation
6. Remove SunPositionerFactory

Essentially turns the class into a very plain C++ one, and expose it to Python like you're doing now.