Protection against division by zero cooling.

These protect against division by zero. Problems were exposed by the lab sim posted in gtests / bugs / Mancini. The sim does get now much farther along but aborts when the line stack becomes misaligned.

The sim is a lab plasma, so it is far from our home base, but we should be able to compute it. It is non-equilibrium. It is unclear why the cooling is zero - perhaps the total is net heating.

The changes seem safe since they are have never been needed and will have no effect in most cases.

In addition, an abort message now have a (helpful) suggestion for how to fix it.