function to change (rdata) or set (csv data) the classes of the required variables in elaborator
elaborator_change_class_required_variables(elab_data, visit, treatment, lab)
elab_data with changed classes (and factor levels)
elaborator_data (requires columns AVISIT,LBTESTCD,TRTP,SUBJIDN,LBORRES,LBORNRHI,LBORNRLO)
desired level order for AVISIT
desired level order for TRTP
desired level order for LBTESTCD