* Remove remaining hard del()'s in our code.
* Replace deleted() and gcDestroyed with QDELETED macro.
* Fix some strange calls directly to Del() (capital D)
* Make Destroy() return qdel hints.
* Make a few of the Destroy()'s more comprehensive in cleaning up references.
* In edited Destroy() procs, converted to use qdel_null and qdel_null_list macros when possible for pretty code.
* Removed unused variable `sleevecard`
* Colored zpipes (like scrubbers or supply for example) would always lose color in game becuase atmospherics machinery sets color = null since most pipes use overlays and the icon cache. For better or worse, zpipes don't, they just use a normal icon_state, so they need color set.
* The proc obj/machinery/atmospherics/pipe/up/process was supposed to be obj/machinery/atmospherics/pipe/zpipe/process.
* Also changed hard coded integers to use constants while I was editing the file anyway.
* Global procs should start with /proc
* Use to_chat() macro
* Added utility to find all z-levels are connected (transitively adjacent) and utility to test if two levels are connected.
