vehicle split z-level support

[mlangsdorf]

add ramp-up > and ramp-down < tiles.

make vehicle_part::precalc into a tripoint, with z-level default to 0.

in displace_vehicle(), when a vehicle structural tile moves off a ramp, increment or decrement its precalc.z as appropriate. when the pivot point moves off a ramp, increment or decrement it's pos.z and decrement or increment precalc.z.

ie:
0. bike with pivot 0, 0 and frames [0,-1], [0,0], [0,1], wheels [0,-1] and [0,1], and unified control system [0,0].
0a. vehicle global pos is [23,24,0] with the wheel at [0,-1] on a >.

1. The bike moves north to [23,23,0] the wheel precalc becomes [0,-1,1], the next is still [0,0,0] and the last one is [0,1,0].
2. The vehicle keeps moving north, so the pivot moves up a z-level to [23,22,1] and the precalcs become [0,-1,0], [0,0,0], and [0,1,-1].
3. The vehicle keeps moving north, so the pivot becomes [23,21,1] and the precalcs become [0,-1,0], [0,0,0], and [0,1,0].

open questions:
what to do about vehicles that are wider than ramps?

ro-o\
|###|  >
|###|  >
ho-o/

center of a vehicle goes up a 2-wide ramp, including the pivot point but none of the wheels.

ro-o\
|###|  <
|###|  <
ho-o/

center of a vehicle down down a 2-wide ramp, including the pivot point but none of the wheels.

sensible solution:
a vehicle can only be split between z-levels while some part of the vehicle is on a ramp. when no part of the vehicle is on a ramp, relocate the vehicle's global_pos3.z to the average of the wheel's pos3.z and then zero precalc.z for all parts.

which may cause vehicles to phase through walls and then suddenly collide =/