docs: def calc_snu(eta, kappa, width, elongation, dist):
      """Calculate the flux density S_ν given a simple physical configuration.
  
      This is basic radiative transfer as per Dulk (1985) equations 5, 6, and 11.
  
      eta
        The emissivity, in units of ``erg s^-1 Hz^-1 cm^-3 sr^-1``.
      kappa
        The absorption coefficient, in units of ``cm^-1``.
      width
        The characteristic cross-sectional width of the emitting region, in cm.
      elongation
        The the elongation of the emitting region; ``depth = width * elongation``.
      dist
        The distance to the emitting region, in cm.
  
      The return value is the flux density, in units of ``erg s^-1 cm^-2
      Hz^-1``. The angular size of the source is taken to be ``(width /
      dist)**2``.
  
      """
      omega = (width / dist)**2
      depth = width * elongation
      tau = depth * kappa
      sourcefn = eta / kappa
      return 2 * omega * sourcefn * (1 - np.exp(-tau))