Change default map-z offset (MAPHEIGHT) to 3

So now 7 z-levels get loaded at once (z-level 0 where the players character is, 3 z-level below and 3 above)
Terrain/vehicles can be seen from above. The starting shelter has stairs leading to its roof. From there players can see the terrain around.
Works with vehicles, too.

make map::copy_grid and map::forget_traps use tripoints as grid coordinates

fix bugs in map::shift when calling copy_grid