Improve the function 'create_specific_linelist'

1) The visibility criterion has been reworked. Macroscopic broadening
   and noise in the observation are now considered when judging whether
   a line is visible or not. Consequently, the qualifier 'epsilon' has
   been replaced by the qualifier 'noise_factor'.
2) The function can now be used to check whether lines labeled as "absent"
   in the input linelist show up as missing lines in the residuals. The
   related qualifier "absent_chi_threshold" has been added.