Converts Step_triggers to Portals!

Implements Portal Functionality to replace the older step_triggers!

Z-level transitions are now 99.9% seamless!

It's self-explanatory, images below in editor:
![](https://i.imgur.com/EBr9mnN.png)
![](https://i.imgur.com/lP0UQxT.png)
![](https://i.imgur.com/c6VX1EC.png)
![](https://i.imgur.com/dfqDz75.png)
![](https://i.imgur.com/pUNVmBG.png)

In-game Version:
To Surface Mining:
![](https://i.imgur.com/aoJIwps.png)
![](https://i.imgur.com/nqVCYV9.png)
To Solars:
![](https://i.imgur.com/FRb6ZKB.png)
To Plains/Wilderness:
![](https://i.imgur.com/Y8kf40k.png)

code/game/objects/effects/map_effects/portal.dm

@@ -9,6 +9,9 @@ Portals do have some specific requirements when mapping them in;
 	- There must by one, and only one `/obj/effect/map_effect/portal/master` for each side of a portal.
 	- Both sides need to have matching `portal_id`s in order to link to each other.
 	- Each side must face opposite directions, e.g. if side A faces SOUTH, side B must face NORTH.
+	- Clarification on the above - you will be moved in the direction that the portal faces.
+	  If Side A faces south, you will be moved south. Dirs are 1/2/4/8, 1: NORTH, 2: SOUTH, 4: EAST, 8: WEST.
+	- You must have 1 tile of 'buffer' space between turfs or you may see odd visual distortions. If things look weird, move your 'portals' back by one tile.
 	- Each side must have the same orientation, e.g. horizontal on both sides, or vertical on both sides.
 	- Portals can be made to be longer than 1x1 with `/obj/effect/map_effect/portal/line`s,
 	  but both sides must have the same length.