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Add freeze FCT limit coefficients option in ZalCG

This allows freezing the FCT limit coefficients once the residual falls
below a certain value towards marching to steady state.

Usage:

steady = true
residual = 1.0e-8
rescomp = 1
fctfreeze = 1.0e-5

Converge to steady state until density residual reaches 1.0e-8 and
freeze FCT limit coefficients below 1.0e-5.