[RM-7695] implement FASTISM

Created on 2022-11-28 08:30:13 by Didier Vibert. % Done: 0

like the fastIgm processing:

• check the wavelength above which the ISM extinction is not applicable (=1.0) and thus if this wavelength limit is not observable at a given z do not loop on all ism coeffs
• if the wavelength limit is observable at a given z, do the chi2 computation on the wavelength range above the limit only once for all the ism coeffs.

This improvement of cpu time, is already done for IGM, since not applicable above 1216, restframe: just mimick the code "fastigm" for ism