add "dump excess" option to gas tank

A quality-of-life feature - add a mode to Gas Tanks where they will always keep a little bit of empty space by dumping 256 units whenever they have less than 256 units of space remaining. The idea is to mimic a real-life [safety valve](http://en.wikipedia.org/wiki/Safety_valve). Prevents gas processing pipelines from getting gummed up if they run out of storage. 

Does not change routing at all, so its possible for a tank to be dumping gas when there are other valid spots to put that gas.

Obviously, this affects balance to some extent (makes it easier to set up a stable pipeline), so if it doesn't fit your vision, go ahead and close.