* source_take: tableof liquid source node names to take
* itemname: itemstring of the new bucket item (or nil if liquid is not takeable)
* inventory_image: texture of the new bucket item (ignored if itemname == nil)
* name: user-visible bucket description
* longdesc: long explanatory description (for help)
* usagehelp: short usage explanation (for help)
* tt_help: very short tooltip help
* extra_check(pos, placer): (optional) function(pos) which can returns false to avoid placing the liquid. Placer is object/player who is placing the liquid, can be nil.