Moved the last bits of logic to do with setting up new particles into their own initialisers (InterpolatedEntityPosition.cs and EntityVelocity.cs). EntityParticleEmitter.cs now *just* periodically creates particles. This should be moved out into spawners soon (handled by ParticleEmitter.cs in the same way as initialisers)

Signed-off-by: Martin Evans <martindevans@gmail.com>