Investigate model stability under zstar vertical coordinates at higher resolutions

I found that DARS2 mesh coupled to OpenIFS Tco319 crashes within a day at 400s timestep under zstar vertical coordinate system. The same mesh runs stable for a month at 600s timestep when using linfs instead. The purpose of this issue is to find and hopefully fix the reason for this unexpectedly large difference in model stability.

Adding @dsidoren 