referential class allows to choose for several parms with checkbox
titleA "character", the title of the box giving the possible choices
labelsThe logical parameters choice
checkedA boolean vector
Objects can be created by calls of the form
new("ref_checkbox", ...).
Other referential objects: charge,ref_choice-method,
ref_choice-class,
ref_coe-class, ref_dc-class,
ref_df-class,
ref_horodate-class,
ref_list-class,
ref_par-class,
ref_parqual-class,
ref_parquan-class,
ref_period-class,
ref_stage-class,
ref_taxa-class,
ref_year-class