Implement a new interstellar extinction law

The following changes come along with this commit:
1) The function 'interstellar_extinction' is revised:
1.1) The so-far used interstellar extinction law by Fitzpatrick
     (1999) is replaced by a new one by Fitzpatrick et al. (2019).
1.2) Instead of reddening an input flux, the function now returns
     only the factor to correct for interstellar extinction.
1.3) Consequently, it is no longer possible to use the function
     to redden photometric magnitudes.
1.4) Furthermore, the function requires now different arguments
     than before (l, E_44,55, and R_55 instead of l, f, and E_BmV),
     which implies that the extinction coefficient R_V is now an
     argument rather than a qualifier.
2) Owing to the new interstellar extinction law, some parameters and
   function names have been changed:
2.1) Parameter E_BmV -> E_44m55
2.2) Parameter R_V -> R_55
2.3) Function fit_theta_E_BmV_R_V_atm_params -> fit_theta_interext_atmparams
2.4) Function fit_theta_E_BmV_R_V_fit -> fit_theta_interext_fit
2.5) Function theta_E_BmV_R_V_fit -> theta_interext_fit