Determine whether thermohaline mixing has problems at highT

This is a problem i see often with models going to core collapse. Models with thermohaline mixing on, around oxygen ignition, grind to a halt. What seems to happen is we have some thermohaline mixing at the very center, the oxygen ignites slightly off center above the mixing. This thermohaline blob then stops the burning from moving inwards to the center and the model then proceeds with tiny time steps.

The solution is usually just turn off thermohaline mixing at late times and as its a slow process its probably not got enough time to actually do anything to the model, but then that also begs the question why its having any effect on core burning in the first place.

I'm leaving this bug here in case any one else runs in this, or is motivated to work out why and whether we need to fix something about the mixing/burning.

Start by reproducing the reported behavior in a model with thermohaline mixing enabled around oxygen ignition and compare it with thermohaline mixing disabled at late times. Investigate why the off-center thermohaline region prevents burning from moving inward and causes tiny time steps. Done means determining whether this is a real mixing or burning defect and documenting or fixing the cause.

Written by the indexing model from the issue text.