Blender Git Loki

Particles: initial support for forces in simulation node trees

The force node can now be used to control the behavior of particles.
Forces can access particles attributes. Currently, there are three attributes:
`Position` (vector), `Velocity` (vector) and `ID` (integer).

Supported nodes are: Math, Vector Math, Separate Vector, Combine Vector and Value.

Next, I'll have to split `simulation.cc` into multiple files and move
some stuff out of blenkernel into another folder.