[RM-8059] TemplateFitting on spectra centered at mean value

Created on 2023-05-12 14:03:43 by Pierre-yves Chabaud. % Done: 0

Could be mean or median value (must be constant).

Needed for spectra with negative amplitude.

goal: remove a constant (mean or median) from the spectra AND the redshifted template on the observed range, before fitting.
The continuum removal method (spectrum.component) could be extended for mean removal, but for the template, since the removal is redshift dependent this is more complex: the removal has to be done after redshift interpolation and after ism and igm extinction.