Add centrifuge

The centrifuge, currently only existing in an MV variety, is a machine
that separates a mixed substance into its constituents.  Currently the
main use is to reverse alloying of metals.  The alloy separation recipes
intentionally only operate on the dust form of metals, making this less
convenient than the original alloying.  It also only recovers metal
constituents, not the carbon that went into cast iron or carbon steel.

This change incidentally generalises the technic recipe and
machine infrastructure to handle recipes with multiple outputs.
As unified_inventory's craft guide can't yet handle that, these recipes
are not registered there.

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