Better reactor meltdown

Better reactor meltdown

Reactor `explosion' now replaces the reactor core with a corium source
node.  Corium is a new liquid, which flows a bit like lava, but has
the additional feature of destroying nodes to which it is adjacent.
It also randomly turns into a solid form, chernobylite, which makes an
attractive building block.  It thus gradually melts its way through the
reactor shielding layers; a meltdown gets worse over time if not cleaned
up promptly.

The mechanism for an active reactor core to damage nearby players is
generalised into a "radioactive" node group.  Corium and chernobylite
are radioactive, to varying degrees.  Players receive a varying amount of
damage from a radioactive node, depending on proximity.  Staying outside
a reactor cube is sufficient to be safe from the active core, but not
sufficient to be safe from a melted core.