WIP adding furnaces to building hacks
This (some day will) implements #1366
How in-progress is this?
As noted in #1366 this lacks the support for machine in furnaces. Adding such thing would be very hard. Features that are left might be valuable. Also as is noted in code - there could be use from adding furnace specific features (afaik only melted metal fractions is unique to furnaces).
Today i've commited last part that was missing. So what is left is testing (can't remember if i've tested this enough) and doc updates (probably to the new inlined docs)
Yeah, partial support for furnaces is better than none. Doc updates would be great. Feel free to merge when it's ready and tested.
I'll close this PR for now, but the related issue is still there and keeps a reference to this PR when we look at it again