[RM-8231] [parameters] refactor linemodel.continuumfit

Created on 2023-08-24 13:10:35 by Didier Vibert. % Done: 0

the @continuumfit@ section is only useful when the parameter @continuumcomponent@ is @"tplfit"@ or @"tplfitauto"@
But the code systematically reads some of the parameters in the continuumfit section.

render the C++ code compatible with that: if @continuumcomponent@ is not @"tplfit*"@ then do not read these params nor use the associated variables.