Pipeworks API change, sorta

Pipeworks API has changed slightly and now requires a bit more to add support to a mod and its nodes.  Nothing much, just a few lines to check for pipeworks' presence and to call the tube routing functions if so.  Full details in the forum post where I announced it:

https://forum.minetest.net/viewtopic.php?f=11&t=2155&p=169628#p169628
