Permafrost refreeze from soil is disabled

[bpbond]

As noted in #722 (comment) -

When the climate cools, new permafrost can form; the carbon for this is preferentially pulled from the thawed permafrost pool, but if that runs out, from the soil pool (i.e., carbon that was never previously permafrost can freeze). This process (i) produces numerical differences in the climate when run globally versus in multiple biomes, and (ii) has no significant (in a science sense) climate impact. So in 0953efd I disabled it, and will open an issue to come back to this later.