[RM-8178] [Refacto] Cleanup snr calculations

Created on 2023-07-11 14:48:29 by Fanny Dufresne. % Done: 100

• The same formula snr = dtm/sqrt(mtm) is used in severeal places and redefined several times -> find the right place to create a method to be used by all
  ex:
• tempaltefittingsolve.cpp (buildExtremaResults, FitSNR calc)
• linemodel.cpp (precomputecontinuumfit)

• Method getFitContinuum_snr() has been defined but unused -> which need ? to delete ?

MR: https://gitlab.lam.fr/CPF/cpf-redshift/-/merge_requests/532