New default vertical turbulence model (Nezu & Nakagawa mixing length model) + new default mixing length model (Nezu & Nakagawa)

Old default value for VERTICAL TURBULENCE MODEL is 1 = constant viscosity which may be quite coarse. Change of default value to mixing length turbulence model (= 2) and in particular Nezu & Nakagawa model (MIXING LENGTH MODEL = 3). So that default value for MIXING LENGTH MODEL is changed to 3 = Nezu & Nakagawa rather than old Prandtl (= 1).