radiation values do not add

[programmerjake]

when using with armor from https://github.com/mt-mods/technic_armor, the putting different pieces of lead armor on in some cases make the radiation value decrease by putting on more armor instead of increasing as expected.

this looks to be caused by:

3d_armor/3d_armor/api.lua

Lines 416 to 419 in 7fc313f

	-- DEPRECATED, use armor_groups instead
	if def.groups["armor_radiation"] and levels["radiation"] then
	levels["radiation"] = def.groups["armor_radiation"]
	end

which incorrectly replaces the radiation value instead of adding.