wet_dry boundary data

[jcwarner]

The main criteria for the wet_dry option is that the free surface can not fall below the sea floor. However, this can happen along the open boundary if users provide a water level that is less than the bottom elevation. To prevent this, I added logic to set_data to prevent the BOUNDARY(ng)%zeta_* arrays from having values less than h along. I am submitting this routine here. Maybe there is a better location for it?