Paczynski Grey Atmosphere
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When ``atm_option == 'paczynski_grey'``, MESA...

- DOCUMENTATION NEEDS TO BE FLESHED OUT

- 'old' description: create an atmosphere for given base
  conditions. inspired by B. Paczynski, 1969, Acta Astr., vol. 19,
  1. takes into account dilution when tau < 2/3, and calls mlt to
  get gradT allowing for convection in atmosphere.