[RM-8208] Add getter for other columns of spectra

Created on 2023-08-09 13:29:34 by Thomas Bedrine. % Done: 100

Currently, the columns other than `wave`, `flux` and `error` are only available before the spectrum is loaded into the C++. Those additionnal columns may be interesting to be accessed from outside of the `AbstractSpectrumReader`, where they currently stay unused after going through any optional filters.