Added basic occupant system

Players can now press E on the exterior/interior to enter and exit.
Velocity is preserved on both but I broke exiting somehow (hopefully
will be replacing with world portals soon thanks to bliptec!). Occupants
too far from the interior are kicked out. Changed interior material
temporarily. Added a dynamic light which doesn't work properly. Added
experimental fix for the lighting (it didnt work). Added more hooks,
notably Think which checks if occupants are valid before calling the
hook negating the need to check it in loads of different places.