Make position property modifiable rather than just replaceable

[bwockner]

Current implementaion of position property of renderInstances and filter requires the user to completely replace the value for that property i.e. instance.postion[0] = newX is not allowed and must be done by instance.position = [newX, instance.position[1], instance.position[2]]

This should be replaced by creating a custom class to represent position which implements the appropriate callback in setattr such that the object is correctly updated