This is a complex combo-box where the spectral model for the sun has to be chosen when 
the constant direction source model is set.
Two choices are possible:
-   Automatic distribution flux sources on all spectral bands : this is the simpler way to do it, a black body model has been coded for the sun. But this is an approximation of the spectral distribution of the sun
-   User distribution source flux on all spectral bands : This option may be more precise provided experimental data are available. It is still reserved to fairly advanced user 
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 SYRTHES version   : VERSION_SYRTHES
 Name help file  : Csm_cmb.html
 I. Rupp, C. Peniguel  -  may 2014
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