[RM-7677] [linemodel] remove absorption line elements when nocontinuum is set

Created on 2022-11-16 17:51:21 by Didier Vibert. % Done: 0

when the continuum amplitude is set to zero (either at the parameter.json level or because of a null-compatible fitted continuum), the line elements with abs lines will have their amplitudes set to NAN, but the linefitting function of these elements will be called anyway. To gain speed, we could skip all the abs line element (one big element in case of line ratios).