           Surface stresses (positive upwards) 
             shape must conform to first two dimensions of input fields

             Units:  Kg/(m s**2) = (Kg m/s)/(m**2 s) 
                               = (Kg/(m**3))((m/s)**2) = Pascals


             Flux should be evaluated at time i   for two-time level schemes
                                   and at time i-1 for leapgrog
             If dtau_du or dtau_dv is non-zero, input stress is modified by 
             the implicit correction 
              