Floating divide by zero in subroutine SettlingSolver

[aoloso]

Code crashes with floating divide by zero error at

GOCART/Process_Library/GOCART2G_Process.F90

Line 1580 in 5d3eff7

dt = cdt/nSubSteps

This is because nSubSteps becomes zero. This seems to be because sometime the array tau has negative values.