[RM-5653] Linemodel: enable ignorelinesupport for fftprocessing

Created on 2020-03-06 14:24:48 by Didier Vibert. % Done: 0

today, the "ignorelinesupport" option for masking the lines from the continuum (to fit independantly the continuum & the lines) is blocked when using chisquareloglambda thus enforcing the continuum orthogonalization

in principle it should be doable in Fourrier too