Add the possibility to compute mass fractions from number fractions

Spectroscopic results based on the fit-function 'stellar' yield
number fractions. With this commit, it is possible to convert
them into mass fractions with propagation of the respective
uncertainties. To this end, the function 'mass_fraction' has
been revised to allow for Gaussian error propagation (and for
the distinction between helium-3 and helium-4). Moreover, the
functions 'mass_fraction_confmap' and 'spectroscopy_TeX_tables_mass_fractions'
have been added. The function 'spectroscopy_collect_results'
has been revised and changed to be more flexibel with respect
to its input arguments. The file "templates/spectroscopy.sl"
has been updated accordingly.