Class enabling to load the list of qualitative parameters and to select one of them. It inherits from 'ref_par', uses its 'choice' method
valqual="data.frame"the list of qualitative parameters
Other referential objects: charge,ref_choice-method,
ref_checkbox-class,
ref_choice-class,
ref_coe-class, ref_dc-class,
ref_df-class,
ref_horodate-class,
ref_list-class,
ref_par-class,
ref_parquan-class,
ref_period-class,
ref_stage-class,
ref_taxa-class,
ref_year-class