Improve coupling between vertical SBM concept and GroundwaterFlow component in model type sbm_gwf:

Model type sbm_gwf makes use of the GroundwaterFlow component (unconfined). The groundwater head dynamics are controlled by specific_yield, this is a fixed value per grid cell. The vertical SBM concept calculates unsaturated store dynamics, which makes specific_yield actually dynamic. Setting the specific_yield dynamically by the SBM concept improves the coupling between the two components.