[RM-7712] implement masked/flagged spectrum

Created on 2022-12-12 11:14:35 by Didier Vibert. % Done: 0

2 use cases:

1. to be able to use the fft templatefitting with flagged samples
   today the client is removing the flagged samples which is incorrect if we rebin the spectrum (for logsampling), or invalidate the regular log-sampliing

1. to carry inside the rebined templates (derived class of CSpectrum) a mask of the lines when fitting the template for the linemodel. This refactoring will make it easier to implement the multi-obs for the linemodel (#6028) to not propagate through the whole hierarchy of calls a vector of vector of masks. EDIT this is not relevant anymore, since the implementation of the maskBuilder class

TODO:

• add a member mask inside CSpectralAxis (TBC)
• implement the CRebin class (and CSpectrum::rebin) with a mask (mask the interpolated samples computed from masked samples)
• implement the templatefittinglog operator with masks
• remove the mask from templatefitting operator methods, use instead the intersection of both masks (the one in the spectrum and the one in the template)