 height of the level where xi is defined
             shape must conform to that of xi
             units = meters

         factor -- real
             used if one wants to modify the units of the flux input 
             The units for flux are [factor]*[xi]*kg/(m**2 s)
                where [xi} is the unit for the field xi
             Thus, if xi = temperature in K, and if you want to input the 
               flux in W/(m**2) set factor = heat capacity per unit mass
               in (J/(kg K))

      