add options in traqsr to extrapolate surface chl data downward

By default, 2D chl data for light penetration is based on an analytical profile from Morel et al. (1988). This formulation is too restrictive and can lead to erroneous chl profiles.

To improve this, we propose to add a new option to initialise chl data in traqsr which consists in extrapolating surface chl data downward (this options was available in old nemo versions and still needed for operational configurations at Mercator).

Proposed namelist option:

!-----------------------------------------------------------------------
&namtra_qsr    !   penetrative solar radiation                          (ln_traqsr =T)
!-----------------------------------------------------------------------
...
   nn_chldta   =      0       !  RGB & 5BD: Chl data (=1) or cst value (=0)
   nn_chlprfl  =      0       !  RGB & 5BD: Chl vertical profile
                              !  0 = vertical extrapolation of surface data
                              !  1 = vertical analytical profile deduced from surface data
                              !  2 = 3D Chl data
...