[RM-6729] [linemeas] should we use improveBalmerFit ?

Created on 2021-10-06 10:19:53 by Didier Vibert. % Done: 100

in the linemodel "free" case, there was an option improveBalmerFit defaulted to yes.

this option has not been propagated to the linemeas operator which thus instantiate an ElementList object with the member @m_opt_enable_improveBalmerFit@ default initialized to false.

Note: improveBalmerFit refit jointly the lines that have superposed emission and absorption components, if the line width in emission and absorption is different enough