Fix peat auto-fermenting bug

Peat should never ferment on its own, it should require face
contact with some kind of soil.  Unifying it with tree growth rate
code caused it to ferment on its own because the assumption that
tree roots contain their own soil and base moisture.  Peat should
only have its own moisture but not its own soil.

This also allows peat to be used as an emergency "moist" node on
its own in e.g. skyrealms for cultivating flora, as long as it's
contained so that only its moisture can leak out but it cannot
be exposed to fermenting microorganisms from nearby soil.

mods/nc_tree/api.lua

@@ -10,9 +10,9 @@ local modname = minetest.get_current_modname()
 nodecore.register_leaf_drops, nodecore.registered_leaf_drops
 = nodecore.mkreg()
 
-function nodecore.tree_soil_rate(pos)
-	local d = 1
-	local w = 1
+function nodecore.tree_soil_rate(pos, d, w)
+	d = d or 1
+	w = w or 1
 	nodecore.scan_flood(pos, 3, function(p, r)
 			if r < 1 then return end
 			local nn = minetest.get_node(p).name

mods/nc_tree/compost.lua

@@ -88,7 +88,9 @@ nodecore.register_soaking_abm({
 		fieldname = "compost",
 		nodenames = {modname .. ":peat"},
 		interval = 10,
-		soakrate = nodecore.tree_soil_rate,
+		soakrate = function(pos)
+			return nodecore.tree_soil_rate(pos, 0, 1)
+		end,
 		soakcheck = function(data, pos)
 			if data.total < compostcost then return end
 			minetest.get_meta(pos):from_table({})