Revise the functions 'evol_track' and 'mass_age_from_teff_logg'

The functions 'evol_track' and 'mass_age_from_teff_logg' have
been revised to run faster and to be more robust:
1) The function function 'evol_track' now uses linear instead
   of cubic spline interpolation to save runtime and to avoid
   numerical interpolation issues.
2) The function 'evol_track' now features the qualifier 'fields'.
3) The qualifiers 'guess_Mi', 'guess_w', and 'guess_frac_age' have
   been removed from the function 'mass_age_from_teff_logg'.
4) Instead of using the fractional age, 'mass_age_from_teff_logg'
   is now using the index into the EEPs (equivalent evolutionary
   points) as fitting parameter, which should yield a smoother
   chi-square landscape and thus improve the fitting process,
   in particular after the terminal-age main sequence.